If it is that size, You should ideally have a 150w Basking lamp in there as that is quite a large area for a 100w to try and heat up i would say.
I used to keep mine in a 4 x 2 x 18" and that needed a 150w, Then at night i had a 10w Blue lamp on the other side come on for 8 hours during the night.
Are you measuring with a digital Thermometer?
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