Hi
I'm about to do the same thing to my boat (CS27). I have insulated past boats and will use the same material as before, 1/2" Ethafoam. Easy to cut with a razor knife, easy to glue to the hull sides as it's flexible, and good closed cell insulation. It's made by Dow, is totally inert and not expensive. It is available in various thicknesses but I've always used 1/2". In lockers you don't really have to cover it. My neighbor (we both live aboard) in a Spencer 35 has most of his accessible interior lined with it. It has been there for about 20 years - I knew the boat and the previous owner when it was done. In the lockers below the seating it is exposed and is pretty easy to keep clean. His quarter berth alongside the hull is exposed as well and for the most part it has stood up well - but his cats have scratched it a bit
. What I will do is glue it to the hull with a good spray glue and in exposed areas cover it with varnished thin wood strips. In cabinets where it isn't seen with the door closed I will cover it with 1/8" plywood either varnished or painted, protecting the hidden side of the plywood with epoxy. It is available at plastics shops in this area and is pretty common. A google search shows it in black but I have only ever seen white. Hope this helps.
Brian