I printed the prototypes in PLA+, but pretty much any rigid filament (meaning, not TPU) should work. I printed them with 5 walls and roofs, and 999 floors, which prints it solid, without infill, because 100% infill can do some weird stuff. The nut goes in the back of the bracket, and should be a fairly snug fit. I used an M3 button head to pull the nuts into their holes. The connector should be flush with the back of the bracket and cap. The easiest way to get everything together is to assemble the bracket and cap around the connector, then screw the bracket to the center tower.

Required hardware:
2 M4x16 button head machine screws  (socket head cap screws will also work) 
2 M4x10 button head machine screws (socket head cap screws will also work)
2 M3x12 flat head machine screws
2 M3 hex nuts