So when I came across the Cush-It booth while wandering around the Fred Hall show this year I had to check it out. What they make is a line of slip-on rod butts.
They use a foam similar to Crocs to make them super light weight and tough, so light they are supposed to float your rod and reel too. They are not marketed for fly rods but the sales people assured me they would work. They even followed me over to one of the vendors that carried their product to show me the correct size to get (The Inshore Model). It did need a little trimming to fit without rubbing the reel rim, but that was easy with a sharp razor blade.
(I trimmed the pink one to fit a different rod and reel - might have fit this rod without modification. Also these are both the same size, just the camera angle makes the yellow one look smaller, but you can get an idea how much I trimmed off the pink one.)
It does not get in my way when casting or seem to affect the rod balance. I got to try it out on a calico today and it felt much better against my ribs and chest than even the biggest fighting butt I own. And it floated my rod, reel and sinking line! (note: It won't fit over this extra wide style of rod butt)
The show had the best price, but I only bought one. I ordered a few more since then from tackledirect.com $13 and if you buy 2 the second one is half off. http://www.tackledirect.com/luna-sea-in ... ellow.html
One more tip: They fit tight and can be hard to remove. There is a hole in the bottom - plug the hole with your palm when pushing it on a rod to let trapped air pressure help stretch it on and when removing shoot some compressed air in there to slide it off easily - otherwise it may pull your cork apart like it did to me.