Gunsmithing HK Shotgun Forend help

[5RWannabe]

i bought a surefire forearm model 616 for my early # 50,000 range HK MIS90 from some guy on ebay. he said this "worked as intended" but he did not have the Spacer Tube that fits between the receiver and the forearm , which also goes around the magazine tube. Surefire no longer has these either. what can i do ... make it out of wood? does any body have the dimensions on this thing or another solution.