Deaner,

I think you are over engineering your boxes. Sounds like you are going to build a tank to contain light. Keep in mind the box is mostly to keep stray light out and growing light in, Not to contain a bomb.

I built all my boxes out of 2x4s and white board. I build a box frame then line it in the white board.
Here are a coulple pics.

Sorry forgot to rotate.

I built mine for around $75.00.