Having done this on Rhapsody, I agree with Russ.
The Balsa core is 3/4" thick. The inner and outer skin is approx 3/8" combined, with an irregular surface on the inside. Use a non water absorbing material for backing plates and you'll never have an issue. Since you selected aluminum and have stainless hardware, put tefgel on the interface so you don't create a battery that corrodes. Better yet - make the backing plates from G10 and that issue disappears!