We ended up machining one for our fittings that worked really well- It relies on the flat surface on the top of our fittings as well as the mounting hole. It takes a single flat head screw to mount, and it puts a drill bushing right over the proper location.
The principle should be pretty easy to duplicate for low-pro's- any of the above would work well, but I would definitely recommend putting a drill bushing in the hole. For those that don't know what that is, it's a basically a press in guide that's the same size as the drill bit, and made of the same (really hard) material. What it means is the drill bit can't cut it and deform the hole.