As a kid my dad tuned me into using JB Weld as a filler to rebuild razor tails. Do it before you get past the last two plies so you have something to hold it in place. Worked pretty damn well, much stronger and more abrasive resistant that typical wood filler, since it has steel particles in it instead of just plasticized particles or wood particles. I would put tape around the perimeter of the tail to act as a "form" for the relatively flow-y state of JB Weld before it hardens.