Shark SUP foldable US fin box and Quick Fix Fin - SUPboarder Magazine
US Fin Box
airSUP US box fins - Fins
4.5" Soft Flex Center Fin – Badfish SUP
Amazon.com : 8" US box center fin Safety Flex Soft replacement for longboard SUP Stand up Paddleboard River Surf Whitewater airSUP AIR7 Skeg with optional 'No-Tool' Fin Screw (Black (easy screw included!)) :
Understanding common Fin Box types. - Liquid Surf and Sail