While I generally support the concept of fabricating your own input panel, going after a piece of steel plate with a Uni-bit, as illustrated above, is a gnarly process and I don't recommend it. At the very least, it should be pointed out that bit should be used in a drill press and the workpiece needs to be
firmly clamped in place. (I can't emphasize that last part enough...don't ask me how I know.

) A radio chassis punch is a more appropriate tool, and doesn't cost any more than the right-sized Uni-bit, IIRC. An easier DIY-er solution is to adapt a plate that's pre-punched with holes for the Speak-ons. Keep in mind how much real estate the L-pad needs behind the plate.
As
@beans-on-toast pointed out, Parts Express stocks a pretty wide selection of pre-punched dishes.