UVB is unnecessary. I do use an old reptiglow 2.0 just for lighting for aesthetic purposes and to see well into his setup.

Cover about 85 percent of your cage top with saran wrapped cardboard cut to fit. That'll hold in your humidity better. Make sure you leave 10-15 % open for ventilation. That should bump up your humidity.

Another option is a good water feature that is deep enough for him to fully submerge. Relative Humidity (RH) is not as important this way.

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Thanks
DW