Well, if we think in terms of finding a 55 gallon tank or having one, it becomes very cheap since the front and back glass on a 55 is 19" tall and just about every MFKer probably has one. For example, I just saw an ad on Craigslist, which I have yet to receive a reply from, for two 55s for $50 each.
If I wanted to squeak every gallon out of this on a budget, I could make twin 4' long tanks of whatever gallons that I wanted with minimal materials. For the average MFKer, turning an old 55 into even two 110s would be exciting. I know it makes me drool. I just happen to have about 6 or 7 tanks that I could apply this too.
The sandwich idea for the plywood between the dimensional lumber makes this build easy for the MFKer with even the most basic skills. So, this is how I see it:
low cost
low skills to build
high gallons
strong structure
adapts to whatever you have on hand (old tank size)
coating of choice (it doesn't matter what you choose, they all work with this)