you need to get more creative.buy a terra cotta planter big enough, make a hole. ~$15.
this isn't africa tho, and they're not in the wild. they're pets. there's no benefit to dropping temps.
also, how does "extra" vitamin D do anything for a nocturnal animal??? there's no basis for that at all, and there is no science proving positive or negative. here is a study done on captive ball pythons showing no correlation between exposure to vitamin D and how well they absorbed calcium (the biological process vitamin D is used for).
i cover 3 sides to almost all my snakes enclosures. whatever sunlight makes it into their tubs is plenty.