For me all I need is the flexwatt, because AP cages are made from 1/2" PVC they hold in the heat very well. But if you need to increase your ambiant temperatures the best way to do so is with a Radiant Heat Panel.
How are you measuring your temperatures? When I have my thermostat set to 100-105 the floor of the cage stays around 91 degrees.
Also remember that the heat has to get through the PVC before the floor of the cage starts to warm up. I would suggest that you set your thermostat to 105 wait 2 hours, then measure the floor temperature.
This is how I measure my hot side temperature with the probe of my Acu-Rite
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