Step 4: Build the separator
The second part is almost as simple as the first. It has a hole for the shoulder bolt hinge, and a slot to catch the magnets. The slot isn't even required, but it helps a lot with making sure you get exactly one magnet without catching magnets on the edges. This is also the motivation for cutting the hole in the back plate sized exactly to the magnets you want to use. For larger magnets, it's less important, but the 1/16" thick disk magnets we're using for throwies are more difficult.
For the hinge pin hole, I used a reamer to get exactly the right size. I drilled the hole to 31/64", then reamed it to 1/2". This results in a very smoothly moving hinge, without any play or wobble to it. I liked the feel of this hinge, and it was part of the reason for choosing the shoulder bolt design in the first place.
The slot for holding the separated magnet is also very simple. I used a 9/16" end mill to create enough clearance for easy operation. Just raise the work up to touch the end mill, raise it another 0.065" or so, and cut the slot. Make the slot a few thousandths of an inch deeper than the magnet, so that everything moves smoothly without jamming when the magnet separates.