Isolating heat gaps on terrarium

I am having trouble keeping the heat in my Heterometrus terrarium. Even with a heating pad I still cannot get the temperature higher then
25-26 degrees celsius. I am replacing my scorpion with a Hadrurus Arizonensis soon, and those need even higher temperatures. I wonder if
anyone can help me find a way to isolate the spots on the terrarium that are most vulnerable for heat leakage. I believe that the most exposed
spot must be on the because of the holes and the fact that heat climb upwards.

That is a lot of volume to heat. The most reliable way, and energy efficient, is to use a thermal battery under the tank. A thermal battery is a high density material that retains the heat and releases is slowly and evenly. As example, a 2 to 4 inch thick 'plate' of concrete with heating elements in it. The down side to thermal batteries is they respond slowly. It can take 48 hours for the ambient temperature in the tank to respond to adjustments to the heating elements.
The best thermal battery operates according to outside air ambient temperature.

Heat leak. Conduction, convection then radiation. Bottom of the tank conducts (Rather minimal probably), the ventilation convects, and the glass does all three. Your average terrarium is a heat dispersal unit.