I Need monopod advice

I already have a heavy-duty tripod for my 500mm f/4 but would like some advice on a good monopod when using a 100-400 f/2.8 IS +2x TC with marginal shutter speed and want a quick setup in a boat or crowded area where a tripod would not be practical. I don't plan to use it very often. It needs to be very compact and lightweight.

Thanks

I picked up a Gitzo GM5561T 6X Carbon Fiber 6 Section Series 5 Traveler Monopod and put a Really Right Stuff High Capacity Monopod head on it.

It's perfect for the 100-400 and the 500mm (I've used it with both).

I also have a 5 series Gitzo monopod. I find it stout and solid. I mostly use it with my 400 f2.8L IS. But, it is great for all my lens/body combinations. An inexpensive alternative is a Manfrotto 680.

Gitzo 2541 for me. I don't bother with a head - it's lighter, more stable and still OK for most shooting.