I'm a bit late to the conversation here, but I bought four of these car seat occupancy sensors - https://www.ebay.co.uk/itm/316523892197 - and installed them between the mattresses, one at butt level and the other at shoulder level on both sides of the bed - and connected them to an ESP32 on ESPHome. Works a treat. They're sensitive enough to momentarily disengage if someone rolls over or shuffles about if you need that, otherwise a generous bounce detection allowance smooths it out.