I went starting from the ground going up, 2” xps foam Then a softer closed foam pad, then heating elements taped with kapton tape to bottom of aluminum 4mm sheet then batteries on top. The second soft layer of foam was there to trap heat against the aluminum forcing it to go up while spreading on the sheet at same time as well as protect wiring of pads and pads from damage. Then two aluminum plates one per long side to act as compression with springs and threaded rods with heating pads taped to outside of this as well. One 48 volt pack gets a total of 16 heating pads all wired in series it spreads out heat with no hot spots and no controllers needed (tried various wiring schemes, I kept burning controllers, relays and even buck transformers…) Ali Baba is just full of garbage unfortunately.
Wired in series no vontroller, relays and even or transformer needed. Battery go between 10c and 20c daily with no controls and power used is 1-1.3 amps or 50-75 watt hour at 54 volts. So 600-850 watts for heating each day (per pack) between sunset to sunrise. Not an issue for a 75 kW bank (21 x 2 and 33 x 1)