Check out
www.campmor.com they sell a sling seat for $20USD that fits any size canoe, and when you're not using it you can roll it up and stow it in a pack or bungee it to a thwart. It weighs just over a pound. If you want something a little more permanent, just bore holes for screwing the clips to the gunnels.
http://www.campmor.com/webapp/wcs/store ... d=12500226
I use a Nashwaak wide nylon strap that has gunnel clips that won't slide around. It is like a kneeling thwart only infinitely more comfortable on my sitz bones. It weighs just a few ounces and takes up very little room in a pack when not in use. Nashwaak may still have one in stock, but they've stopped manufacturing. You can see a picture of it a
www.nashwaak.com and use it as a guide to fabricate your own.