The industry standard surfboard center fin box. Also known as a "Bahne Box" it's typically used in Longboards, Single Fin surfboards, Egg and Bonzer Shapes, and as a Stand Up Paddleboard fin box.
You can install a 10.5" center fin box in a quad fin setup to make it a "Versaplane" where the fin arrangements are endless...

 

This 10.5" Center fin box is installed after the seal coat using the

 

Specify Black or White color when ordering

Dimensions: 10.5" long x 1" deep

You can also make your own centerfin router install jig templates below:

Print both pages full scale, tape together, transfer the pocket outline to 1/4"- 1/2" thick plywood 

10.5" Center fin box router template for use with a 1/2" diameter router bit with 1/2" pattern bearing on top (, )

10.5" Center fin box router template for use with a 1/2" diameter router bit with 5/8" Outside diameter router collar  (, )

10.5" Center fin box router template for use with a 1/2" diameter router bit with 3/4" Outside diameter router collar  (,)