I ordered this one in a 20mm and modified it to fit the 18mm slot then flare out to 20. It's very hard to adjust, but once it's done it won't have to be done again. Looks really sharp, and was cheap. https://www.amazon.com/gp/aw/d/B017R1E9P6/ref=pd_aw_vtph_107_2_tr?ie=UTF8&refRID=RYRC8E3J9TGAGQJSY8FS