Dog water bowl, 25 watts of heat. No glass, so dogs can't possibly get hurt from a broken aquarium heater.
I would recommend a bowl warmer. This is what I use for my outdoor pets:
K&H Pet Products Thermal-Bowl Heated Cat & Dog Bowl https://www.amazon.com/dp/B07HMKXQBR/ref=cm_sw_r_cp_api_i_qGYIFbD2B12Q0