I use the large version of these Thermophore devices for my lower back. It generates moist heat and has the safety device of needing a hand to hold the “on” button, preventing you from falling asleep and parboiling yourself. They are expensive but I have had mine for over 15 years and use it almost daily.
“Hold switch down to start moist heat. Let go and it turns off.
Designed for the neck, sinus or TMJ area. It applys moist heat therapy to specific joints – such as the elbow, knee or ankle. Comes with a long Velcro fastener which allows adjustment for a snug fit on any small area.
Original Thermophore Automatic Moist Heat Pack is designed with a special hand-held switch. Just press for heat, release to cool and alternate as often as you like. If you happen to doze off and your hand relaxes, the unit will cool as a safety feature.
Features
Size: 4 by 17 inches
Weight: 2 lbs
Cord Length: 10 feet
Maximum Temperature: 165°F
Electrical Specifications: 110 Volts; 44 Watts – 60 HZ A.C.
Recommended Application Time: 15 – 20 minutes at a time.”
You could only use two at a time unless you have someone there willing to lend you his or her hand.