This is what you need. Place it under your tub. I usually elevate my tub a few inches above this pad. Remember you don't want to go over 82 degrees Fahrenheit.

https://www.amazon.com/TESLUCK-16-5x11inch-Waterproof-Temperature-Adjustable/dp/B07XBF9LPZ/ref=mp_s_a_1_15?crid=1F71VABJKJVKM&keywords=terrarium+heating+pad&qid=1677521732&sprefix=terrarium+heat%2Caps%2C196&sr=8-15