Me personally, I would just shell out the money for a pro-made shock tower brace. I wouldn't want the back of my car looking like a big hack job, plus, if you buy one, you know without a doubt it's gonna work.
I don't see why you couldn't make it work. Stinger's right (as normal), go with square or round metal though. Also consider reinforcing the shock towers, over time it might put alot of extra stress on the several welds holding the towers on.
EDIT: Are the shock towers in a hatch the same as the ones on a coupe?