if you want mirror (which your fish will probably hate..) you can get a light styro covered plastic that they use for bed canopies... its somewhat durable...
I would avoid spray paint and just go get some oil based black or blue paint
or just buy the sheeting for 10 bucks
75g w/ 30g sump, 160# LR
, 60# sand: Inverts =18 turbos, 30 nassirus's, 40 blue leg's, 5 emeralds, 1 CBS, 1 skunk cleaner, 1 brown gulf shrimp? and various other LR
hitch hikers yet un discovered or un-ID
Fish=1 yellow tang, 1 yellow headed jawfish, 1 pink/blue shrimpgoby, 2 perc clowns.
Future fish = 1 sixline, 1 lawnmower blenny