So I went ahead and ordered the aluminum sheet for the sides (going to split it myself). Onlinemetals.com had the best price, also seem to be the most reputable, plus they accept paypal to boot. I had it shipped to work both because it's $2 cheaper than residential, and because I'll need to cut it anyway (there's a nice big bandsaw). $26 for 12" x 36" x 1/16" thick 5050 aluminum aint bad.
I also decided to paint the trough parts. Even though you'll almost never see them under the apron, I just didn't like how bad they were pitted. I debated it because I was worried the ball would start flaking off paint, but I did buy appliance paint which is partially epoxy. If it's hard enough for appliance abuse, it should handle a shiny steel ball in a home envoirnment.
I also ordered this handy little tool I had no idea existed!
I should have figured something like this existed. Basically it allows you to make a 90-degree joint in wood by clamping them in a perfect 90-degree vise. This will really come in handy for the backbox, not to mention other stuff. I'll probably use dowel pins + screws to make a nice strong joint.