I purchased these Knurled thumbscrews from Amazon. They were silver M4x20mm.
We will use the hairpin as the spring clip to keep the thumbscrew in place. You will need to open up the hair pin a bit, cut it off with the dremel and bend it like so.
The idea is to make a slot in the thumbscrew to JUST be flush with the hair pin when inserted in the side like this.
You will need to grind down the front of the thumbscrew like this with your dremel tool. This front pin section needs to just fit into your blade hole (smoothly fit without slop but no force needed to insert). The hole that used to be where the original blade retainer pin was that you removed to disassemble it.
Grind the front and slot into the side of the thumbscrew at these distances.
When settled into the plastic base, the thumb screw should NOT show the slot when its all put together. If it does, your thumb screw will not sit flush on top. Too low and you will not get enough pin protruding to retain your blade.
In the plastic base, you will grind a groove that the hair pin will snugly fit into. This will keep your hair pin from moving out of position when its all closed up.
The last step is to replace the short phillips screw that was under the middle of the plastic base with a set screw so it does not interfere with the hair clip once assembled.
Put it all together and close it up. You should ONLY see the ground down pin showing through the inside.
You should be able to pull on the thumbscrew and release to have it be pulled back into the base.