6 section 15mm Picatinny riser, originally designed for the Vortex Venom red dot sight. It will work with any accessory meant to mount on a Picatinny rail.

Required hardware:
#8-32 x 1in low profile socket head cap screw https://www.mcmaster.com/93615a324
#8-32 x 1/4in narrow hex nut https://www.mcmaster.com/90730a009

Other #8-32 hardware may work, but will not sit flush with the sides.