If you're getting a drill press and they're just square plates (I'm assuming you're getting ecocast or tooling plate for the ground surfaces to put your bearings on?) then a bit of engineers blue, set square, steel rule, centre punch set, scribe and you've already got the hammer!! Mark up the holes, centre punch them, drill on your drill press, tap using your drill press as a guide to keep 90° (search youtube for videos) and bob's your uncle you've got a drilled and tapped plate.....

Alternately search for local machine shops who will mill the edges to your desired shape and drill and tap for you....but will be expensive!