It was a few years back but someone on this group posted pictures of a system he had set up. IIRC, it involved a deflated inner tube, possibly folded in half so you are sitting on a "C" with the open part facing forward, connected to a bottle of compressed air and a valve. When you were ready to get out you slowly opened the valve, the inner tube filled and boom! (I mean, voila!) you were several inches higher. Very basic but it worked or at least he claimed it did.