dlockh4258,
Have you thought about just getting a boxblade or rear blade that you might be able to use for other applications? You can probably get an entry level unit for just about twice what a weight box will cost, and it'll increase the utility of your tractor.
About the only real benefit of a dedicate weight box is that it generally takes up the least amount of space behind the tractor, so the tractor is very maneuverable. If that's isn't an absolute requirement, look for either a rear blade or boxblade. And the heavier the better. You might be able to pick up used units at an auction for a decent price.
Another idea: 3 pt. carry-all forks, and make a box to fit them filled with concrete or concrete blocks.
Ahhh....it's so much fun to spend other people's money. /forums/images/graemlins/grin.gif
Good Luck,
~Rick