There is only one best place.
Dead center on the bottom of any tank.
However, I faced with the same problem as you (Most likely) I urgently needed to heat hydie oil and water on my pride and joy.
I installed a 3000 watt stick-on heater on my hydie tank (About 100 gallons), a 3000 watt heater on my water tank (150 gallons) and a 3000 watt heater on my oil pan.
With the exception of the oil pan....the hydie tank and the water tank heaters were installed on the sides about 6" to 8" up from the bottom on the side.
Amazingly, it works but is predictably inefficient. The top 1/4 is warm.
If you insulate the top of the tanks ... about half the tank is warm. The bottom is cold but doesn't freeze. It absolutely makes a huge difference.
I put way too much heat on my pan heater and as the pan is ribbed, the stick on pad did not adhere very well. 12 gallon oil capacity .... I would have been very happy with only 500 watt.
Haven't been up close to a TC, I'm wondering if there is a weld on coupling on the tank that would allow for an in tank heater????