Hmm... A turkey fryer has WAY too much BTU's. You'd be building a crematorium for pigs....heh. I think someone mentioned a replacement side burner found in some gas grills. That would be more like it.
The old insulation in there, fiberglass would probably be OK, unless ya go with the fryer! I decided not to insulate mine, but was going to use mineral wool type stuff, used for some firestopping applictions in construction.
The reason i decidced against insulation is water. You have to cut a vent in it, and I doubt the unit is watertight from the outside anyway. Think big soggy moldy mess inside the walls.
Far as racks go, you can get square replacements too. I had mine made out of expanded metal. I have to sandblast
em before use and season them, and KEEP them seasoned, but no big deal for custom fit racks.
Also, remember not to build a bomb in case your flame goes out. Always worried a bit about that, without a pilot/thermocouple/regulator system. Don't most of these types have this built in? Hate to see a nice rack of ribs on the neighbor's roof, eh. 'Round here if they end upon MY property, I'm gonna eat 'em.