I used a mod for my Ender3 Pro Printer, however my printer did not have the necessary threads in its 20x20 Aluminium Profile. Thus I remembered the deformable plastik rivets in my car and designed this rivet for M4 holes. 

It is intentionally oversized, so the thinner outter plastic will deform and create a snug fit in the M4 hole. In my case it has a clearance (h) of 3mm between the head and the actual fitting hole. Feel free to remix your h - length as needed.

I recommend printing upside down, with support for the head itself. A support blocker next to the actual rivet helps as well. I am printing it with 0.08 layer height, to get as many layers as possible for a better fit.