Does anyone know someone that either makes or sells one of these inserts? I have recently shaped a fun board that is going to have a futures quad set-up and a futures longboard single fin box. I will be riding the board mostly as a quad, but also as a single fin when conditions suit. I also have a set of twin fins that have a small finner that goes with the set. And would like to buy one of the pictured inserts to make this possible. Does anyone have any info about it? Any leads would help. Thanks

Like Decrep said

Just a standard US box backwards.
Chinook make the best one but any longboard box will do (as you don't need windsurf board kinda strength.)
Cheapest would be from shapers.com.au probably. Just remember to sand the bejeezus out of it with 80grade to rough it up, lest it don't stick