Originally Posted by airhead
I have seen some awesome seats made of close cell foam and plastic covering that come out looking like a Paco Pad.
Try EVA foam (Ethylene Vinyle Acetate). It's used in a great deal of applications for padding and even flotation. It comes in various densities and thicknesses. Look for EVA foam for athletic padding as you can sometimes find pads that come with softer and harder densities layered together.
Then just go the fabrics store, get some nylon (gore-tex like) material, and put on your sewing hat. I'd go with a porous fabric to allow water to evaporate rather than plastic to keep the water out (it'll eventually get in). The major difference between this application and the Paco Pad is packing size as the Paco Pads will pack much smaller.