Where can you find un-warped wood? Not at a big box store, that's for sure. Lumber yards are usually better.
I think I've read on here somewhere that 1" of wood has an r-value of 1. So it's not too good. But the air gap between the two collars will actually increase your r-value, so I wouldn't worry about the air gap. I wouldn't worry too much about insulation, I'd be more concerned about an air-tight lid. That will help prevent moisture from getting in.
If you don't want to stain it, a couple coats of polyurethane will help protect the wood from moisture/mold.
Looks like a great start, nice work.
edit - I happen to have 3 stainless, 1/4" bore, 4 1/8" shanks from micromatic that I didn't use for my project, I'd cut you a good deal on them, you might be able to insulate with that length...