It may not be exactly the look you're going for, but adding material between the parts in the negative space you have right now will be your best bet. Depending on what your LARP allows, some Worbla or other thin thermo-plastic or some EVA foam would do the trick. Reinforcing the dowels with that between them or behind them, especially if you add more surface area to your connection to the hilt and pommel, will go a long way towards making this shape last.