Thermal grease or thermal paste is, well, a paste that’s very thermally conductive. It fills the gap between two pieces of metal, since each piece of metal has its own imperfections that create small air pockets between the two materials. The paste fills in those gaps. It’s used in computers to transfer heat from a processor to a heat sink, and for this application really anything will work fine so here’s some that’s pretty cheap. If it’s a hard case battery and the hold-down still holds it in without the foam, then rip that sucker out. For my hard case 3S batteries I got the battery expansion kit(Traxxas 3725X) which isn’t strictly necessary by any means, but does give way more space for the battery which made taking it in and out much easier.

Battery expansion kit: https://www.amazon.com/dp/B000XQ5CFE/ref=cm_sw_r_cp_api_glc_fabc_0OEaGbHKRYS0H

Some decent thermal paste. It’s not amazing stuff but it’s plenty good enough for this application, and cheap: https://www.amazon.com/dp/B07PQTT3WZ/ref=cm_sw_r_cp_api_glc_fabc_aREaGbEK1J11W?psc=1