Hmm, too thick? Maybe depends on the plastic type a bit. I’m trying to remember what I’ve done different. I guess maybe try slowly shaving it down with a sharp knife was how it works best. I definitely have been able to do that carefully when it gets close to the end. On my last one I first cut it off leaving just a little and then actually smoothed the whole fingerboard out with a file and very fine sandpaper AFTER joining them together. Made for a very good fingerboard.