If you follow the directions on the pad, (not putting any weight on it, and checking it for deterioration periodically), it's pretty safe. Pads designed for human use tend to be held to pretty high standards. You just have to be sure you don't have one with an automatic shutoff. It's not for long-term use over years, anyhow, but there's no reason to believe that an undertank heater is a safer device than a human heating pad. For a few months in quarantine, I don't see that it's dangerous.
(I've used the things for myself on a regular basis, as I have trouble regulating my body temperature due to CFS, and they are surprisingly sturdy).

Using a thermostat with a high temperature alert or high-temp shutoff is always a good idea with any heating device anyhow.