So HD has luan ply that's water resistant. Supposedly it's the adhesive between the layers that's resistant. Though I need to do a little more research this could be perfect. It's less than $12 for a 4' x 8' sheet which is better than the $200 for spanish cedar. And if the adhesive truly is water resistant and it's not a problem for off-gassing or fumes, then I wouldn't have to put the coats of poly on the inside of the exterior box before lining and seasoning. With the size, I can't fit it into my finish room and it would take a while to dry. I couldn't do anything in the shop that would generate dust while it dried. If this works out I could have the humi ready to season by next weekend!