In the heel and fin box area I'm using divinycell foam for reinforcement. Gluing the fin box. In the foreground is a gun sight I built to check fin alignment. The putty was a bit thick and I found I'd be more likely to twist the box with the acrylic in itself than change alignment. The box has to be close to perfect when you stick it into the cut out.
Most instructions recommend using an old fin to check alignment.
I can't imagine putting a short wave fin in the box will allow you to check alignment accurately nor change it if it's off. First session in the vacuum bag. I'm gluing the divinycell for heel reinforcements here with 3.7oz s-glass patches underneath the foam. Turns out the water jugs dented the foam underneath the heel reinforcements a bit. This is the board with all inserts and reinforcements glued in.