Holt Allen's Sliding Gooseneck - Square Base 7mm Groove has an 7/16 in. (11mm) wide x 13/32 in. (10mm) thick backing plate that is snugged with a rotating knob, which is below the gooseneck pin. It the slider fits into a 9/32 in. (7mm) groove. Holt also makes a sliding gooseneck that fits an 8mm mast groove (HA41143).