Curtis, try a couple of different types, but I've found the garden variety hobby shop brass to work well. Aluminum may or may not do what you need, but it depends entirely on the stock. See what's available in your area and find out how it works is the best I can do.
To get 90 degree corners, set up a couple of blocks and a vise. Use a radiused corner on one block to form the bends. Make sure to keep the legs parallel in both planes. You will have to decide if milling the slots is easier before or after the bending is done. Figure on doing a couple of tries until it is just right. tim