The easiest way I found to do this was to grab a can of spray adhesive, its like 10 - 15 bucks, you can go to any dollar store and grab an emergency blanket for like .50 cents, they are 4x6 sheets of Mylar 1mm, spray a corner of the box, attach the Mylar to that part, then while holding it away spray the rest of the area and then flatten it down....it's pretty easy and you could do the whole box for maximum efficiency of your lights!
Just remember with your fans, cause i made this mistake the first time, make sure you have a strong exhaust....most important, make sure your intake is sufficient for your exhaust and make sure the exhaust is at the top....as heat rises......I thought my exhaust at the bottom would have been strong enough but I was wrong and had to move it.......took some tweaking to get it all right. Also when you tested it, you'll need to leave it for on a couple days to get a true reading of the temp, the plants emit heat too if Im not mistaken....this is what the cops see with the infrared, I believe there are only two plants in the world to do this but don't quote me on that.... all I know is my box ended up being hotter than I first had anticipated.....but all in all your on the right track!! Grow em up!!