this might be a better option it has a thermostat.
https://www.ebay.co.uk/itm/Universal-Motorcycle-ATV-Carbon-Fiber-Seat-Heater-Warmer-Kit-Heated-Pad-DC-12V/333113246970?hash=item4d8f18c4fa:g:N1MAAOSwkmJch2lS
25watts isn't that bad when water heater pads are 100w. i maybe could leave a air gap...