This is a hold down system to clamp PCBs. I use it on a Genimitsu 3018 to mill PCBs. It seems to actually work pretty well, and as a bonus there's a void underneath the PCB, which helps when drilling holes.

You will need:

• M5 T-nuts (these came with my 3018)
• M5 Bolts (40mm)
• M3 Nuts 
• M3 Bolts (12+mm)

Print out 2x of bottom clamp, then mirror it along X and print out 2 more. This will let you form a square out of the 4 bottom clamps. Also print out 4x washers, which are used on the M5s.

Using pliers or whatever method you prefer, shove a M3 nut into the opening on the bottom clamp. This is a captive nut that will hold down the top clamp.

Now print out 4x of the top clamp. Use a M3 bolt to secure to the bottom clamp and you can mount your PCB in there.

I have printed with PETG / PLA, both seem to work just fine. I mill 100mm x 70mm PCBs but since these slide around you can mill all kinds of sizes.