They have a bunch of logical requirements such as a roof that affords entry access to clean and maintain the hut as well as a single defendable access point, and a channeled entry to limit size and entry manner. They are usually multi level and have insulation. Reflectix is the norm in the wooden structures. Unfortunately, cats love to shred it or if you get bio issues, it ruins it and often can ruin the whole hut.
Lately I've been making the huts lighter and more portable such that the volunteers, often not as young or spry as they once were, can move them easier. This includes using lighter materials such as thinner plywood sheathing and plastic corrugated roofing vs asphalt shingles. I also put lawnmower wheels on them and feet such that fewer if any cement blocks are needed to get them up off the ground.
I've been thinking though that having an inner, sealed plastic tub liner and then insulation between that and the outer structure would be a better idea. This would keep the insulation away from the cats and intact and would also make cleanup easier inside, given that it is plastic. Worst case, you replace the liner tub/bin all together.
Recently, I built it. It was far more complicated than I first imagined and far more intricate in construction. However, I think that it is pretty sound and should work well. I used a simple Rubbermaid storage bin which is cheap, effective and easily replaceable.
|Bin with divider shelf and lower entry tunnel in center|
|Two layer foam and Reflectix insulation separated from cats|
|Bin fitted inside mating to entry chase|
|Foam insulation over the top|
|Finished product ready to go onsite|