YOu can use a holesaw, dremmel tool, laser cutter, plasma cuter, jigsaw (with the proper blade, but it does cause wavy edges), or a hole punch.
The best method is a hole punch. These can be expensive. Basically nothing more than a cup with a tight fitting disk, you drill a smaller hole large enough to slide a bolt through, then tighten the two pieces together. I'm thinking of making one in the machine shop.
What's the frequency, Kenneth?