help with good cheap mylar? or somthing similar for grow cab

I'm building a grow cab and would like to hear some peoples advice on some cheap ways to get light to reflect. I'm not too big on painting white but if someone has done so before and know a good paint let me know I need atleast 50sq ft. Of coverage ill post some pics of box its about half done right now. Also I'm planning on keeping it in my garage and I live in FL so the humidity is pretty high would I be ok with good vent? Temps in middle of day in my garage is around 85 hopfully ill be ok cuz my wife won't let me grow inside

The horizontal braces can come down as soon as the rest of my walls are up bit i was thinking about pouting them back in straight for help with LST and support and look a little better than it does now a little more symmetrical if you will haha. I could use some pointers on a door also the opening is 3' wide 4' tall. I was thinking makeing a wall 1/3 of the opening then the door being the other 2/3 as a 3' wide door is a little big and heavy if using wood witch I would rather do but anyways any suggestions would help thanks oh and one more thing I just though is put out there I haven't spent a dime on this cab (yet) the whole frame used to be a queen size box spring