Hi Luke, FTE located in Florida can supply you with exactly the correct size 500 mount you are looking for. Heres the website url -
http://www.franktiano.com and phone# 863 607 6611. The T-500 mount used in my MB339 (see pic) will work great on the Roo, Just drill mounting holes into base of clamp and screw to the molded in Roo mount.