Just recently I bought two scrap seats, ripped the foam in half on one of them, glued the pillion half to the other seat and covered it all with a bit of £17 vinyl from ebay. It added a good 2" to the seat height and is a lot more comfortable for me. Granted it looks utterly shit, the riding position is a lot different, there's much more wind blast on my chest at speed too, but my legs are great now over long distances.
If I did it again I'd cut the outer edges off the seat so it feels a bit more like the front half, but I'd definitely reccommend you try it. Cost me £48 in total.