I would go more than just the corners and support the trim all around the base like a metal stand does. Its not like he would be using 1/8 ply its hopefully 1 in and what little area is sticking out i dont think much bowing will happen JMO ;0
Smaller tanks 5s 10s can be just suported on the front and backas you can see in the link.http://newenglandguppies.org/FishRoo...s-driscoll.htm
Anyone can sail a ship in calm waters its when the storms come up shows what one is made of.ODAAT
This is meant to give a general idea as to how I added support for the plywood extending off the back of the stand. The sketch makes this look flimsy, but the actual thing seems darn solid. The sketch also gives the impression that the corners are drastically unsupported. While the actual setup has the corners bare, it isn't as bad as this makes it look.
So here is the general idea of what I did today. Like I said, the tank is only 2 inches too deep, so the dimensions here are not exact. 1/2 in piece of ply sits on top of the stand, extending about 1/2 inch out on the front and sides, and about 4 inches off the back. The 2x4's are screwed into the ply from the top. There is also plywood securing the 2x4's into a fixed position at their bases, to prevent them from ever wobbling side to side. It feels sturdy with the tank sitting on top, have yet to do a fill test.
EDIT: I did the sketch because its dark in the garage, but heres a crummy picture just to see. You can see the screws that go through the ply and into the 2x4, and the rim of the tank straddles the two screws in each.
most hardware stores will cut the lumber if you bring in the specks it costs more, but will save time if you only have a hand saw. that 20 is about how much it would cost if i had to have to store cut the 2x4's for me. and no i did not skin the stand. its just a basic stand.
So the other HD near me will cut for .50 per cut, after the first two. I am kind of reluctant to go that route though. If I do, could I just use those steel corners to make sure everything is lined up at 90 degrees?