Well if you do not have a EDM or a broach in your basement
I have drilled the slot with a 1/32 undersize and then file the corners in, this works if not a great deal of precision is needed, but if you are careful it can be that to.
BTW, if the large center bore is already there then the drill wont get it but taking your time you can file it. You could plunge mill the slot and then file.
Ken