I've got a strip of this in both forearms and thighs:
https://trooperbay.com/yellow-foam-costume-or-armor-fitting-strips
Super Glue and Zap-A-Gap (hate that stuff!!) was just too messy to attach them with, plus they didn't hold (neither did E6000) so I used adhesive industrial strength velcro. Stick one piece inside the armor and the other to the foam strip, press them together and voilà. Hasn't budged once.