that's a good question.
i am somewhat reluctantly going to try 6 mm black plastic sheeting doubled up on itself. my suspicion is that it will not be sufficient to block enough light, but i've seen people on other forums claim success with simple 6 mm black plastic sheeting. so who knows.
my failsafe is what i have indoors for all my little grow cabinets: blackout sheet. $6.99 a yard at joanns, would need 24 yards to cover the walls. unlike the plastic sheeting, there would be no worries of tears due to fatigue from rolling and unrolling every day, and no light gets by that shit.
actually, come to think of it, that would be $168 worth of fabric and a shit ton of sewing, whereas 9 more 4x8's would only be $131. i could buttress the sheets of plywood against the walls with scrap lumber i have laying around, too.
oh, decisions, decisions, decisions.
and for anyone out there keeping track of my original budget of $500-800, i have good news. i initially set aside $400 and am only at $222 so far, so if the black plastic sheeting works, i will come in significantly under budget. if not, i will still come in at or around the $500 mark.*