inexpensive chrome paint sucks balls. It isnt nearly chrome and will reflect like like shit.
Go to a plastic supply house, ask for 1/16th inch thick polycarbonate "mirror" and use that.
Silicone it to whatever youre using as a box and call it a day.
I thought long and hard about this problem the other day and I couldnt think of any reason that people would use the mylar unless they are being cheap.
the plexiglass mirror will sit flush against the surfaces, siliconed on and be water proof making for nearly impossible odds of developing mold.
Its mirror so its reflective, wont tear and is easily cleaned up.
But hey.........its just my .02