I want to put quad fins with a center fin on my wake surf board and when I went to route the center fin the template was not flat on the board because of the double concave I put in
The trick to routing a centerfin box into a deep double concave tail is to simply shim the router template to level. Depending on the curve of your concaves you may have to shape the sides of the finbox to the shape of the concave after the resin has cured.