If you have access to a drill press that's a very good start
I made my first set on a drill press with a ruler and some bits
My second set was with a mill with a digital readout so it was easier.
But you could do it all by hand with a drill, just would take some patience and steady hands
If you are just doing bar with ring terminals I would do 3/8" minimum
It's not a lot of material to grab on, not that it should be able to hold hundreds of pounds but it should be a solid connection