I have a purchased, and one homemade queen castle. Their weight has me thinking that rigid insulation could be used to advantage. I would still use individual wooden inner cover pieces (possibly thinner than 3/4" boards) with Masonite or luan dividers. I picture the "box" and outer cover being made from the insulation.

A slatted, wooden bottom would improve the durability of the assembly, if it can be attached securely.

I would consider building a modified version of the "Coates Nucs" as well...

Does anyone have experience constructing assemblies with rigid insulation?

What is the most effective adhesive to use?