I haven’t gotten much further than just getting these straps and doing a mock-up on a board, these things look promising. The Onetigris one definitely looks larger and more durable, but the other one is made of an elastic type material which could be good as a final over wrap to further hold everything down. Here’s a couple mock up views and links.
For scale
Concave creates gap under strap?
Maybe splint foam can fill gap and help keep batteries/box in place?
Temporary bottom mount? Instant velcro test board.
OneTigris Tactical Headrest Cover… https://www.amazon.com/dp/B07KSYBXW3?ref=ppx_pop_mob_ap_share
TFY Vehicle Headrest Hook & Loop Fastening Tape Strap (Black) - Accessory for TFY Car Headrest Mount https://www.amazon.com/dp/B00CE5YJH4/ref=cm_sw_r_cp_apa_i_8MSaEbHTT1P4K
Dynarex 3528 Rolled ActiSplint Single Pack, 36 https://www.amazon.com/dp/B00GNGU2PI/ref=cm_sw_r_cp_apa_i_5NSaEb0TQBTYS