Synonyms for Gusset (same or very similar meaning)
WordNet sense 1 (a piece of chain mail covering a place unprotected by armor plate):
voiderWordNet sense 2 (a metal plate used to strengthen a joist):
gusset plateWordNet sense 3 (a piece of material used to strengthen or enlarge a garment):
inset, insetFrom the ODE community, based on WordNetadd/edit