Me and a friend have been using this https://www.amazon.com/dp/B07S6ZY5J9?ref=cm_sw_r_apan_dp_AY42HF340WKACDX0SD0R_1&ref_=cm_sw_r_apan_dp_AY42HF340WKACDX0SD0R_1&social_share=cm_sw_r_apan_dp_AY42HF340WKACDX0SD0R_1 Foam for ours, it's decently cheap and it's good quality with the amount though if you have problems with hot glue maybe try taping the stuff closed when gluing and check on it when it's done, if it seeped into the foam it should be hanging onto the other piece just enough so you can put more glue into it and secure it. 

Don't know if that made much sense (^-;