I forgot to post this: the machine needed to bend the aluminum is called a brake. It works by clamping the sheet metal in place and having a chunk of metal fold up to bend along a straight line. To bend 1/8" aluminum that long, you'll need to find a pretty heavy-duty one, probably a hydraulic one.
Which makes it good that you decided to make it out of a single sheet There's a machine that does the cutting there too: plate shears. If any of your friends has one of these, they must be VERY into fabrication: those tools really belong in shops. And to cut 1/8" aluminum, it'll have to be a hydraulic one. The shop at my college had one, so maybe if you can find a friend in a local tech school or community college, the shop supervisor might let you cut to size for a nominal fee or even for free.
Otherwise you're stuck using (in order of preference): a plasma cutter, a cutting torch, a big bandsaw, a hacksaw blade clamped in pliers, or a dremel. You're most likely to find someone with a cutting torch, but be careful if you do because it'll leave a really jagged bad finish on the edge that'll need more work if you don't want to cut your legs on the bench as you sit on it.
You might do well to think of using thinner material, or just fiberglass :P Plywood or fiberboard wouldn't look too bad, but I'm no expert in sealing them to prevent rot.