20mm Riser for RMR style red dots. It uses QTY 2 M3 nuts and M3x10mm bolts to mount the red dot to the riser plate. For the picatinny slot, it uses a M5x30mm through bolt to secure it between the slots. 

I would use some thread lock on the M3 bolts to make sure the dot doesn't lose zero over time. Print in a high temperature material if desired, no supports needed! I printed mine in PLA, dealers choice.