lol in the "old days" a bud of mine would always steal my old dutch potato chip bags I had no clue wtf he was using them for until I finally figure it out..lol

He used them to line the inside of his grow boxes not many people know but they were also made of Mylar also

How hot does it get in the box only turning it on every couple hours I'd think would get too hot in there

The box I made is 18Dx22Wx33H I have 1 55W CFL thats hanging & adjustable and 1 120mm PC fan in the top corner & a smaller 3" hole in opposite bottom corner & I'd have to have the fan on the entire time to keep it @ 77*
I lined mine with tinfoil but doubt I'll use it the closet its in is perfect for what I want anyhow

My babies now are about the same age as yours were when you started this post..
If mine are the same size as yours in 2 weeks I'll be happy..lol