I did a search, and did not find an answer to this, which actually surprised me. I can't believe I'm the first to answer this question.
I am making templates for shallow mortises (1/8 deep by 1/2" wide by 2" long) using a standard Porter Cable guide bushing, not a bearing bit.
The width is not a problem as the mortise is close enough to a standard router bit size and I use a another standard router size for the OD of the guide bushing. So far, so good.
The length is a bit fussy. I have a written offset table that I use to add the offset to the template, which gets me pretty close. I usually make the templates out of scrap 1/4 MDF and make the template in my router table (using a plunge cut dropping the MDF on the spinning bit), but I usually need to run a whole bunch of test pieces (too long, too short, just right) and I either have to re-run them through the router table if its too long, or use a rasp on the slots to slightly lengthen them. I'm experienced, but plunge cuts on a router table still make me nervous, and sometimes the MDF moves, so I sometimes I get a sloppy entrance point.
What would be perfect would be some 1/2 baltic birch ply cut and glued into strips to form the slot the guide bushing rides in, but getting that to glue on on the edges firmly has been a problem. I've considered biscuits, or slots and spines, and pocket screws too. I've also considered 1/2" pine drawer stock (which would edge glue nicely) but I'd rather use plywood scraps if possible.
Of course, I cannot edge glue MDF.
How do you guys make your templates for guide bushings?