Thank you all for the feedback!
I decided to bite the bullet on the outer sheets, and skimp on the inside.
Our tear will have baltic birch-marine grade on the outside walls, AC ply in the middle, skarfed and insulated, and white birch to finish the inside walls (got it on sale for $30 sht
We plan on using fiberglass to finish the outside - by hand, not sheets (we own Tony's book, and will be following his advice!) .
I had not heard about the glass foam, and will research that
Also, have not heard of Russian birch nor apple ply. I will look into those options as well.
Funny, wife just talked about scavenging armoires for cabinet parts!!
Ted