Although there are loads of great magnet setting tools out there iv always had the issue of they can drag the magnet back out of the hole is the fit is a little loose, so i made this one you release the magnet you want to place by sliding the switch back. There is another magnet inside the tube that is moves via the switch, once its fare enough away it no longer holds the magnet to be placed. I use a spring to keep it biased forward but it will work without one will just feel a little more rattily. 

this one uses a 5mm x 1mm magnet on the slide (it doesn't need a strong magnet to work).

if you want larger just scale up all the parts.