Hi guys and girls,
This may interest those with the Cokin Z-Pro system or those looking into filter holders.
If you can't be bothered reading, skip to"Using a screw in polariser and two filters" 
After reading countless posts asking for advice on which filter system one should purchase (usually between Lee and Cokin) I decided to write what I have learned after going through that process myself.
I have been trying to aquire a filter set over the past few weeks and the research was doing my head in. I wanted a filter that I could use a circular polariser with and up to two neutral density filters without vignetting with my wide angle lens. The options are few and far between with the most reccommend kit, by Lee, being near impossible to find in stock.
After a lot of research I decided to try the readily available Cokin Z-Pro filter holder. I also wanted to compare Lee filters with Singh-Ray so I purchased a Lee soft grad set, a Singh-Ray 3 stop hard grad and a Singh-Ray 3 stop reverse grad filter.
First impressions with the Z-Pro holder were excellent. I assume Cokin has re-designed the Z-Pro holder because after reading many (older) reviews complaining about the quality, sturdiness and rotating mechanism I can say the quality is very good. The adapter attaches to the lens easily and it sits in the holder very tight holding the system in place. There are three pins that keep the adaptor in place and they are by no means a design flaw. They work very well to keep the adaptor centered and the adaptor ring slides very smoothly while you rotate filters.
The holder as assembled from the factory has three filter slots. However only the two farthest from the lens can be used with ND Grad filters; the first is designed to hold a drop in circular polariser. The front slots held both Lee and Singh-Ray filters and are a perfect fit for both.
Vignetting
On the weekend I packed the Z-Pro holder with ND filters and head for the beach. I wanted to use my B+W screw on circular polariser and experiment with one or two ND Grads. After setting up the kit I immediately noticed strong vignetting with the polariser and two filters in place. I was expecting some vignetting, but not this much. Even worse, as I rotated the filters the edges of the holder covered the corners of the frame! I tried the holder with only one filter in place and still got vignetting and of course the holder still covered the corners of the frame when rotated. The only option was to remove the screw-in circular polarising filter. Without the polariser, the vignetting reduced greatly, especially with only one filter in place, and the holder no longer covered part of the frame. This is quite acceptable for many situations. However in some cases I would want a polariser along with GND's.
When I got home I removed the screws from the Z-Pro system and took the front clips off (This alone should prevent the extreme "vignetting" when rotating the filters). This got me thinking and I tried a few more arangements until I figured a way that would allow the filters to be held much closer to the lens. It turns out I am not the first person to think of this 'reverse design', however I couldn't find anywhere that shows how to do it so I will show you what I did below. The below arrangement allows me to use two filters and a screw in circular polariser without extra vignetting. As drop in circular polarisers are very expensive I am sure there are many people that would appreciate being able to use their screw on polariser with two more filters.
Before I figured out this reverse design I had finally found two shops that had Lee equipment in stock, one had the foundation kit and the other had the wide angle adaptors. PM me if you would like their details. My thinking was Lee's wide angle adaptors will help with the vignetting. I am unsure now whether the Lee or Cokin system will be better for wide angle problems. Once the Lee system arrives I will be able to thoroughly compare both and can post back if there is any interest.
Lee vs Singh-Ray
I have not had much time to compare filters however both appear to have extremely little color cast. There is a very small difference in color between the two but I would not be able to tell the difference in photos. The Singh-Ray filters are more expensive. The Singh-Ray filters come with very nice leather pouches. The lee filters come wrapped in a nice, but less convenient microfibre type cloth. I find the Lee '10 Filter holder' to be the most convenient and safe way to transport multiple filters. It is excellent. After being exposed to salt on the beach I found the Lee filters were somewhat easier to clean than Singh-Ray. Lee do not make the ingenious reverse grad filter. If I were to buy a complete set knowing what I do now, I would buy Lee filters exclusively with the exception of the reverse grad from Singh-Ray.
Using a screw in polariser and two filters
For those that skipped to here, below is a modification to the Cokin Z-Pro system to allow the use of a circular polariser and two filters with a vast improvement in vignetting. It may even be better in that regard than the Lee system. There is one major problem when using the reverse arrangement with a screw-in circular polariser. Once you attach the Z-Pro system to your lens you can not rotate the circular polariser independantly from the filters. I found a (very crude) way to get around this problem shown in the next post.
I used the following: The card (credit card type card) is make-shift until I go to the hardware store - you may think of something better. For the first wide angle arrangement, you do not need anything other than the Z-Pro holder and adaptor ring.
With the factory arrangement, the adaptor ring slides into the back of the Z-Pro Holder
I attached it to a B+W Kaesemman MRC Xs-Pro (SLIM but with front thread) Circular Polariser, and had minimal vignetting
This is the configuration. You swap the adaptor ring clamp to the front side of the holder. In the image below you can see the adaptor ring clamp (which has a greater curve than the filter slot clamps) is moved from the bottom to the top of the holder and reversed.
Adaptor ring is now on the front side of the holder (Right side below)
You can see how close the screw-in polariser is now to the filter holders, reducing vignetting
This is how much closer you are able to place a filter to your lens! I am waiting for the Lee kit to see which is closer. It seems the Z-Pro may be closer after the modification. Look at the distance from the red ring to the filter for easy comparison. The problem now is you can not turn the circular polariser independently of the filters. My next post shows a crude, make-shift way to overcome that problem.
P.s If you arrange your Z-Pro system this way you may want to shave down the two pins at the bottom of the holder as they are very close to the filter even when retracted. I sanded mine back and glued them to prevent them from scratching filters.
Thanks for the kind words.
