Find yourself a Metal Supermarkets, they are worldwide, their crop (offcut bins) have various pieces and thicknesses of aluminum for around 5.00 per pound. Aluminum is fairly light. Go for the utility grade (5052/3003 either will work, 5052 being a marine grade, 3003 being a ageneric grade, avoid anything stamped 6061/2024/7075 they are expensive aircraft grades. Besides the 5052/3003 will take a bend if you need to put one on the plate. The other grades will snap unless its an extremly mild bend over a greater span. Hope this stears you for the aluminum choice. Warehouse Manager Metal Supermarkets.
P.S.: Aluminum can be worked with some woodworking tools as well as metal working tools. Non magnetic, and easy on electricity. Does not rust either. Hope this helps.