Joe,
You are well on your way. The sheet aluminum can be cut and bent by any local sheet metal shop. All you need to do is make drawings after you decide how you want it to line the box and take that and the sheet aluminum to the shop and they can do it easily.
I would suggest you make drawings and then lay out the pattern for cutting on paper so you can use the sheet of material most efficiently. If you want to do the sheet metal yourself you can cut it easily with a 3" X 1/16 abrasive blade in a die grinder, but it will not look as nice as sheared pieces. You can bend it yourself to make the flanges also, but nothing looks as good as brake formed flanges as Harry has suggested.
FWIW you are in the middle of an area that has a lot of used machinery dealers and a lot of those specialize in sheet metal equipment like shears and brakes. They can hook those up and let you play if you offer to pull a trailer for free for a local delivery.