ECB 3D Printing has created our own design to the Invisible Body mount. Ours uses a Ball Stud to locate within the mount. The studs come in m3 and m4 thread sizes. 
 
Our mounts are a little different than standard magnetic mounts. Standard magnetic mounts are flat and hold well when pulled but when slid sideways they slip right off. The ECB 3D Printing Invisible Body Mounts are created to reduce the chance of that slip. Our mounts hold the sides of the ball stud and also uses a magnet to keep the ball stud in place when pulled. Also using a round surface like the ball end allows the connection to the body mount to be at an angle as needed. 
 
These are sold in pairs coming with 2 ball studs and 2 mounts to be glued to the body. You can use these with normal flat magnets, having the ECB 3D Printing Invisible Mounts at the front or back of the body and then the flat magnet mounts on the opposite end. 
 
The ball studs are easily added to the end of a standard plastic body post. Just cut the plastic post to length and drill a hole. When using a m3 ball stud you will need to drill a hole close to 2.5mm (.098") to screw into and with the m4 ball stud it will need a 3.5mm (.138) hole drilled. The ball studs can be threaded up and down for precision height changes.