I made a jig.
It has stops for the baseplate of my old Bosch POF router, so no guidebush and you can see what you're cutting. You set the depth in from the edge with the router's fence.
It works OK, but you still have to trim the corners and cut the recesses in the casing. In my case I screw the jig to the casing (designed it to be able to do that), but I don't know about the commercial ones.
I'd give mine about 5.5/10 - better than nothing but far from a perfect solution. And it took an age to make. I'd improve it a bit if I made it again, but even so, probably not worth the fuss given the small number of doors I do.
I wouldn't want a jig where I couldn't see what was happening, which is usually the case with the guide bush variety.
E.