I used CAD to design a jig that slides onto the trim lumber and is secured with screws using index holes, cut with CNC. The index holes also match the final screws that will hold the caprail trim in place. Practicing on some scrap lumber, it took three different versions of the jig to dial in the tolerances to ensure a snug fit.
|Jig attached to a practice piece of wood.|
|Two ends ready to be joined. Note the biscuit on the top piece.|
|Test pieces joined.|
Next is to do this on the real PlasTeak caprail trim.