Where the heck do you live? I have to drive 2 hours but a sheet of 1/2" plex-g is less than $300 where I get it. You're getting ripped off unless you're looking for Polycast.
The Arkema (plexiglas brand) non-printed mask is $240/sheet for 1/2" (0.472). That's the "general purpose" grade - same stuff as "G", made in same plant, they just call it "import" instead of "domestic" even though it's all imported from Mexico. Even the black 1/2" is $260. I can get up to a pallet drop shipped to my front door for $100. Shop around dude. Sabic Polymershapes.
The Cyro (acrylite) tells me that 72 x 24 with closed top is .944 or 1" material. But according to James (or someone, somewhere in this thread) they tend to overshoot with a closed top and undershoot with an open top. So you could probably do 3/4". That being said I have seen ten e cor tanks 66 x 24 with 3/8" (polycast, so 0.375 not 0.354) and sea clear/etc tanks with 72 x 24 made from 1/2" but oh man you should see that 72 x 24 sucker bow. I would do minimum of 3/4" if it were me.
BTW 3/4" Arkema brand cell cast clear "general purpose" from Sabic <$400 sheet (approximately. Was $345 but 6% price increase just went through and I never have bought that stuff anyways so I don't know for sure what it runs now)
As for the cutouts, I would think that 4" perimeter and 2 6" crossbraces with 2" radius corners (that's the most important part) would do good. That's 3 16 x 16 cutouts, roughly. But that's a ? for James as I haven't built tanks that large and have no personal experience...
