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.
Jerry Garcia 2007 Reviewed by Jerry Garcia 2007 on . My first grow Thought I'd put this here so maybe someone can see it and give me pointers. My plants are about 3 and a half weeks, they look good, to me anyway. Just started them on 600wt mh, and pure blend pro grow, mixed to about 1/4 strength. One plant is about 4 days older than the other 2. Temb keeps hovering around 88-90, I know thats probably too hot. I don't know when to feed them again, I didn't use much, 2 look dry already, but I felt moisture a couple inches down so will water tommorow. Got a ph Rating: 5