asesino i heard of people just usin white bed sheets would it be ok 2 use those?
It would depend on how thick the sheets are, and again I'd imagine they would get very hot and absorb all the heat as opposed to deflecting it onto your plants. Alot of light would seep right through the sheets and out of the aquarium. Also, when you flower your plants on the 12/12 cycle you need complete darkness. If you had just a sheet shielding your plants from light (assuming your aquarium is a clear colour?) some form of light would be bound to creep in. Mylar is very cheap to obtain by the roll from hardware stores. Or even just take 4 thin sheets of plywood, paint them flat white and line the inside of your box with them.
Bearing in mind your aquarium would also need some form of ventilation I'm not sure how you'd get round that. A wooden cabinet with a door is easy to cut holes in for fans to draw air in and out, but a glass tank? Hmmm..I dunno..