You can go buy emergency survival blankets that are 1ml mylar at 'Sprawl-Mart' for $2 in the camping or hunting department. 54"x82" is what I bought. Tape it up and there ya go. When you want to take it down if you ruin it...it cost ya $2. I was using foil and this stuff looks like it will work way better. At least it looks way better. I could go on and on about reflectivity and does it really make a difference. (there's your opening HD)

And since you brought it up...I've seen some pretty unreal grows done in grow rooms that were a loonnnggg way from having optimum reflectivity on the walls. And these people were into growing big time. For big profits. Yes they used big lights with lots of lumens, but no more than anyone would use in smaller personal grows comparitively. They just had 20'x20' rooms so needed more of 'em.

But I use the stuff. And before that I used foil with all kinds of terrible light 'black holes' all over the place but they didn't go nuts with a massive growth spurt when I put the mylar up.

I think painting the walls white and everything else in there would work perfectly well. I doubt very much if you'd see any difference at all (except in your billfold) if you used the most reflective mylar out there.