I suggest 2mm steel sheet. Sketch out your design and find a local metal working shop. They should be able to cut and bend the sheet in one plane (direction). Drill holes yourself. Cheaper than you probably think, you could probably get multiple blanks for small extra cost. New tariffs will make it a bit more expensive in some places.
Cut steel edges can be a bit sharp so take care and round them off with a metal file and some sandpaper.