Well, after making two or three trips to the hardware stores trying to scout out material, I finally checked online, and found some racks a lot cheaper than what I saw the other day, including one at REI. So I went over and picked one up for $20 at REI. Still a bit of work to mount it to a bike that wasn't intended for it, but it'll work out all right.

One of the materials that I overlooked at the hardware stores was metal conduit. They had 1/2" conduit that would be sturdy enough and is fairly light, and cost was like $1.67 for a 10' piece. Meanwhile, aluminum and steel angles and similar sections were around $8-15 for a 4' or 6' long piece.