Macklots get dehydrated pretty easily in low humidity - moreso with the young ones.
When you live in an area with low ambient humidity, it is a battle. The easiest mthods to increase humidity are to use a plastic storage box filled with damp sphagnum moss (partially over/under the heat source) and to decrease ventilation in the cage during seasonal low ambient relative humidity.
Regarding undercage vs overhead heat - here is what I have found:
When I use undercage heat, the air retains better humidity but the substrate (and therefore snake) dries quickly. In that case a substrate that holds moisture plus a hide box helps. This is where the humidity box works well.
When I use overhead heat (radiant heat panel or ceramic heat emitter), the moisture in the air dries out quickly but the substrate holds moisture longer. A humidity box also works well here too. I don't use a lid. I use a cat litter box filled to the brim with the moss. Moisten as needed.
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Mark