Bicycle bag for a long lens

I'm looking for a bicycle bag for a longish lens & tc (15" long interior needed) to put on my rear bicycle rack. I've researched the holster-type bags which would seem to be ok for that use (no rings for attaching to the bike, though). I wondered if anyone else had any ideas about that? I don't go anywhere rough on the bike, just my local rail trails and parks, and I find my backpack throws my balance off too much (small woman).

So good padding needed, but I can add a pad underneath if necessary.

I'm kind of surprised the bag manufacturers seem to have missed this market - I can't find anything made precisely for this use - so what do you all use? Any ideas appreciated.

Cheers.

This works well on a kayak for a gripped 7D and 100-400L with hood attached. It probably is not what you are looking for but it has rings and may give you some ideas.
http://www.overboardus​a.com …slr-camera-bag-large.html

Thanks C Scott: a friend of mine has one of these, unfortunately doesn't work very well on a bike. Previously I used a toploader on it's side for my 70-200, worked quite well but not long enough for the 300.

What type of rear rack do you use? Need just padded or padded and waterproof?

Try thi link. I strap this to the from of my handle bars via Velcro straps for my 70-200 when I go out.

We cycle-camp.
In fact were out last weekend.
I did a timeplapse and you can see me packing up my hammock/tarp/quilts we sleep in while
in the woods.
The racks on the forks might be of interest to you, they can hold almost anything, crazy things
like a growler of beer! Or in your case a PVC tube padded inside for a lens. and you can carry one
on each side
https://www.flickr.com​/photos/medicineman404​0/23394715849/in/datep​osted/

Here is an image extracted from the timelapse that shows these two racks on my forks.
Hope this might give you some ideas-

IMAGE LINK: https://flic.kr/p/BKFu​Tf

Interesting fork racks! Is it commercial or custom? Think op has rear racks only.

Oh I'm sorry didn't see he was racking solely on the back.
The racks are called Anything Cage by Salsa.
The fork is from Muru Cycles in Australia.

Thanks for all the ideas, folks - this gives me a lot to work with!

Medicineman, you are really a pro at cycling with gear - I'm afraid I just ride an old clunker on short rides of about 15-20K max, on nice flat rail trails, lol. By the way, great images on your Flickr! I hope I can get birdie images as good next year!

I'm willing to mod or upgrade to a nicer bike if I can get some sort of reasonable protection for the camera - and the fork packing rack sounds like a better system for weight distribution, so thanks for that idea to both of you.

Thanks again, and Merry Christmas to you all!

Cheers, Chris.