I would just put a keel strip on the boat so you can just enjoy paddling it without a lot of fuss. Just epoxy a strip on the back of the boat that tapers from nothing to about 1 1/2 inches high by 3/4" wide - about 1 1/2 ft long.. You can shape it a bit once it's on the boat and seal it with epoxy. That will solve your tracking problems. There's hundreds of links here on seats from a slab of foam to all out molded seats.

I agree with Bill, just keep it cheap and make it work for you by looking at other boats and what they have for accommodations.