I purchased the box aluminum from a local welding shop, mainly because they were less expensive, closer to home and they carried a heavy-duty grade of aluminum. The stuff that Home Depot carries is a bit lighter, but it's still more than sufficient to hold a couple hundred pounds of gear. Just be sure that Home Depot, or Lowes, has the correct length. They used to have 6-foot lengths, but the last time I was there I seem to remember the pre-cut length was 5-feet. The most expensive part of the project will probably be the detent pins, which are also available at Home Depot and Lowes. I had a bunch of them here, most of which were used for holding the Bimini-top in place on the boat. If I recall, they measure about 1-1/2 inches long and they're 1/4-inch diameter. You may find them cheaper at an online marine store.

Gary