I found slotted screws but what I wanted was something with an oversized head that I could use to index and slot myself. All the ones I could find, including at Brownells, Numrich, TOTW, etc, didnt have enough head material for the process. In fact, the one from csmc doesnt have quite as much head to work with as I thought at first. But I think I can make do.
I've only got one screw to do, but I'm thinking that to get it perfectly lined up, I'll need to cut the slot myself. I guess I would always try a slotted screw and see if I get lucky with the alignment, but the truth is that I am interested in learning the process anyway, and this seems like a low risk project to try it on.
I want the screw slot to be at least as straight as this (just the example I found online):
Another option would be a brass threaded insert into the wood with a machine screw. At least it would be easier to find a blank that way, I think.