You could use any varnish or polyurethane that has UV inhibitors. A spar varnish is formulated in such a way as to be UV resistant and resistant to large changes in temperature and humidity unlike varnishes intended solely for indoor use. So in essence, marine varnish and spar varnish means the same thing. The term “spar” is also sometimes applied to non-varnishes. An example is Helmsman Spar Urethane which is fairly inexpensive and would also be an option
As for whether to use paint or a clear finish, do you like the appearance of your hull bottom? Using epoxy and a clear coat like varnish or urethane will result in a fairly transparent finish. If the bottom of your boat is something that you would just as soon cover up, use an opaque finish like paint.
I have the CLC “Creature Comfort” seat shown above in two kayaks. It has enough flexibility that it would conform to the bottom of a hull pretty well and could be secured in place with heavy duty Velcro. It is comfortable. But it would provide a very low seating height unless you blocked it up on something. Unless you have a canoe with a very low sheer, I think you would find the gunwales in your armpits and might find it uncomfortably low for fishing. It is also fairly pricey and would cost more than making your own seat.
As for minicell you can cut it and shape it in a variety of ways. A band saw is excellent if you have one. I have used coping saws, keyhole saws, fillet knives to rough cut thick plank. Others have used serrated bread knives or electrical reciprocating carving knives. For shaping surform tools and various grades of sandpaper work well. Yes, fine fitting is by trial and error. Rough cut the seat to be slightly taller than you want it to be at the deepest part of the hull and then trim down the bottom at the sides to match the curvature of the hull. You can also contour the top of the seat to match your rear end if you want. It really is not that time consuming but you will generate a lot of minicell “dust” that will want to stick to your clothes, so do the work outside, or somewhere you can easily vacuum up all the dust. Use DAP Weldwood contact cement (flammable variety) to glue minicell panels together and glue it to the hull, but you need to apply at least two coats and preferably three to foam because it soaks up the adhesive.