The old dipstick heaters went away because they weren't a good idea. Because there was only a little bit of surface area of the dipstick actually in the oil to provide the heat, it put out a very high concentrated heat that was bad for the oil. If you found one of these and put it in your water tank, it would probably melt a hole in the plastic tank wherever it touched it.
I'd use a silicone heater pad. You can find them in various physical sizes and various wattages. They're simple to install. Clean the bottom of the tank very well and apply a thin coat of high temp silicone from any auto shop. The hardest part is holding the pad up tight against the bottom of the tank while the silicone dries.
Silicone heater padsBill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.