I've found that 10g tanks fit pretty nicely on a 2x12.
You can simply get a long one (or a couple), and cut it up for 2 uprights on either end, and as many shelves as you want, and put simple dadoes in the uprights to support the shelves.
I don't know how many tanks you could support end-end on a 2x12, but I'd probably want to put a support piece in between each tank.
I've got one I built for a 20XH (same footprint as a 10g), that I built out of 1 2"x12"x10'. Probably cost about ~$15 or so, and a couple hours worth of work one evening. I've only got one tank on it now, but if I wanted, I could easily stick one on the bottom shelf as well.