after I cut out the square n the back for the slots I measure all four sides and cut out a paper template of the back. After that I center the key slots on the template, then mark the outer line of the template on the piece. I cut the excess off, sand the back down a bit, the sand the edges on a bit of a bevel to fit. Then some extra thick ca glue to hold it in place. I've been asked to post a tutorial with pics, and I completely spaced it out on this last helmet, but it's ok, i've still got 3 more to do very soon. That tut will be posted, fear not, gentle reader.