Behind the balast is an extractor fan...and the covering isnt foil its the stuff you rap people up with who have hypothermea etc.....
ems blankest, not much better, even glossy white paint would be better then that. get flat white room paint for your box and you are fine
those blankets reflect only about 45-55% of the light back since the are single layered only, you can look thru them really
make cool shades for sure
the crinkles of it will give you heat spots in the grow, burning the plants! so i guess you are roasting a turkey there, i will come for dinner
if you want cheap mylar go to a dollar store and search for it in the wrapping section, the sell it as gift wrap
but i can ensure you that flat white paint is just fine and less hassle since you have to get the mylar perfectly flat and un-crinkled to the walls which his a damn hassle!