Long or DIY fin box

Hello - 

Looking for a 12.5+" /320+ mm  fin box for foil boards.  If anyone knows where to get please let me know.  Thinking that they proprietary now I’mk looking for anyone with knowledge on how to build one with say carbom or liquid plastics.   THX!





I home made some by cutting and gluing two 8" boxes. Work well. 



15"er at shaper supply