Check out this new punch that does slotted holes in aluminum and spring steel. I helped to develop this tool for the company that makes them. I always wanted to make slotted holes in my body braces but I couldnt see getting an iron worker just for that. Instead we came up with a tool that uses a regular press to do the job. It also has changeable die sets that can do round holes or slots.
It really works well and makes professional braces that are adjustable.

http://www.tubeshark.com/images/mini-punch-flier8x11.jpg