I've got a stand for a 10gallon that I built about a year ago. Going to do another one (hopefully) in the next month.
You can do the whole thing with only a circular saw, and you don't even really need to measure.
go get a 2"x12", at least 42"+2*(desired height of stand), and get it cut in half (approximately) at the store. a 2"x12"x8' should be good for about 24" or so high stand, get a longer piece if you want more height (although, it's going to be a bit unstable with such a small foot print and large height - I put a hook and eye bolt to attach mine to a wallstud).
once you get home, but the two halves lying next to each other up against a wall or something. Leave enough leftover to serve as a shelf on each piece (at least 21-22", you want to have enough for the length of the tank, and around ~3/4" extra on each side), and cut off the pieces for the uprights. keep them butted up against the wall or whatever, and do two simple dadoes across each end of the pieces (set the saw blade so it only goes ~3/4" down, attach something to guide it on each side of the circ saw foot, so it can only cut a track 1 1/2" wide, and just repeatedly cut across that area until you have a notch. A chisel helps a lot to remove material, but if you don't do one, you can do all the work with a circular saw). Make sure you keep the boards butted up against something solid the whole time, this ensures everything is aligned and at the same height. Also mark one end as either top or bottom, so you don't get them mixed up.
Then cut the remainder to fit in the dado and have enough room to support the tank. (also cut both pieces together, butted up against the wall). set one end piece on the ground, put some glue in the notches, and push the shelf pieces in (might take some sanding/pounding with a mallet). Get both seated well, and then add glue to them, and put the other end piece on. Once it's all glued and pounded together, throw a couple screws into each shelf, just to make sure everything is nice and tight and held together. The dadoes and narrow width of the stand help to resist some racking, but it wouldn't hurt to put some diagonal pieces across the back.
Here's a couple pics of the stand I have, everything is a bit of a mess, (it's even worse now...), but it should give a general idea of how it goes together. Second pic is a close up of the top dado joint, to try and illustrate what I was trying to explain up above. You can see the marks from the 1/2assed sanding job I did
It's fairly easy to do, takes only a circular saw (cordless drill helps with the screws...), doesn't need much skill, and is pretty cheap (can probably get a 2x12x8-10 for less than ~$20), and doesn't take much time (did it in a couple hours after work one evening). Just make sure you leave enough room on the bottom shelf so you can work on the 10 gallon easily. probably need a 2x12x10'.