Modular Cup Holder - M5 hardware

Use M5x 30mm screws at the smallest (I used these) to connect the two halves

You can use M5 heat inserts (I like the CNC kitchen ones) in the clamp side of the connection to get a VERY strong connection. Holes are slightly undersized, so you'll need to use some elbow grease to get excess plastic out of the way. 

If you use a screw longer than 30mm, you can just use a nut and washer to accomplish the same thing.

The magnet holder fits a 25mm or smaller disk magnet (I used these). While the tolerance is already tight enough to give you some friction to keep the magnet holder in place, I put rubber bands on either side of it to ensure it stayed snug.