Building a safe low-temperature heat box : heat sources
It will be difficult to build something which is intrinsically limited by its heat output to a low temperature but still brings the contents up to temperature quickly. 

Making some assumptions about the box:


Wall thickness: 3 inches
Surface area: 4460 square inches (random 7 cu ft freezer online, inset dimensions by half a wall thickness on each side)
Insulation R-value: 1 (m^2 * K) / (W * in)  [estimate based on Wikipedia numbers]
Temperature difference: (100 - 72) = 28 deg F = 15.6 K
Power to maintain temperature difference =
\$
\frac{\text{surface area } * \text{ temperature difference}}{\text{wall thickness }*\text{ R-value}} = 
\frac{\left(4460 \text{ in}^2 \left(\frac{25.4 \text{ m}}{1000 \text{ in}}\right)^2\right) (15.6 \text{ K})}{ \left(3 \text{ in}\right) \left(1 \frac{\text{ m}^2\text{ K}}{\text{W in}}\right) }
= 15 \text{ W}
\$


To guarantee that the temperature inside the box passively stays around 100 F, we would have to limit the heater's power to 15 W, which isn't much compared to the thermal mass of big hunks of wax. If the box is better insulated, then the power must be even lower. This also assumes that the temperature inside is completely uniform. If not, the power must be lower still to guarantee no hot spots.

I like the aquarium heater idea. It limits the possibility for overheating the wax both by the safety features of the commercial heater, and by the boiling point of water.