You should not need a humidifier. Build the plexi box in the bottom, then you don't have to squeeze it in. This is going to be a very damp enclosure, if your wood is already not treated, it WILL need many coats of polycrylic or you will continue to have wood swelling problems. I didn't realize 3 sides were glass. You may have to build in some corner supports out of wood to secure the branches to. Brackets and screws should work OK. Have you tried looking online for frogs? I can't believe no one has frogs for sale.