I've include a photo showing a seat on risers. Basically, before you canvas, you install two hardwood rails to the interior of the canoe hull. Screwed in from the outside, one screw per rib. You mount the seat to the rails by simply screwing it in from the top. As the seat is frimly attached to the riser, and the riser is firmly attached to the hull, it acts as a thwart. Some may not like using risers, but its use in canoes goes way back. You can see it in the Rushton Indian Girl, and many other canoes as well. Tremblay always used it. It also reduces the number of holes in the gunnels and allows the seat to be mounted lower, which are nice benifits, too. I've also included a photo of the caneing pattern used by Chestnut/Peterboeough, just for the record.