Fin box placement

How do you determine the correct placement of the center fin box (10") to ensure that it is level from side to side so to make certain that the fin will be verticle and not be tilted to either side. It would be a bummer to put in a fin box only to find out that the fin is leaning over to one side after everything is said and done.

When installing box, put a fin in it, tape it off so resin does NOT seep inside, and use tape from rail to tip of fin to other rail to hold upright.