UTK Ultra-Soft Heating Pad for Pain Relief, Heat Pad for Back, Neck,Knee - XXL [36"x24"], Auto Shut Off, Adjustable Temp and Memory Function https://www.amazon.com/dp/B07WLY6WNC/ref=cm_sw_r_apan_i_KNG7DD70JWEMNYVNYGMF?_encoding=UTF8&psc=1

Stays on up to 4 hours at a time and this thing covers my entire back and shoulders.