Keep it simple
You might find an older raft with a removable floor. I see them around for $100 or less. These have aluminum "keepers" on the sides, and in the floor joints.I see these rafts all the time for $100 or less.The aluminum could be cut to fit your raft. The edges are rounded to reduce chafe. You could also glue an additional layer of something, maybe truck innner tube,cut to size, to the floor.I replaced the floor in an older Dynous, and used a material similar to Starboard.It is actually used commercial butcher shop cutting board.I sanded the surface smooth, and milled the edges to fit the aluminum keepers.Link below for photos.