Get a cheap scraper and go to work with a grinding bit, and make a profile like the below. Wrap teflon tape or something around the blue bit (to keep it from eating away the edge of the board), and slightly sharpen the edge of the red bit. You'll be able to scrape your own rabbet pretty surprisingly fast.

You could also try multiple passes with a slot-cutting (t-profile) router bit with a guide bearing on it. I'd probably do the scraper unless I had a bunch to do, way less of a chance of an 'oops'.
edit: Actually, if you have a router table and fence, you could mount the slot-cutter in the table, set up some stops, and rout it safely. Advance the piece carefully against the rotation of the bit, and you should be fine. If you don't have a router table, I'd go with the scraper over hand-held.