The lamps are sucking out the humidity for you. Snakes don't need a basking light or any other special lighting. Do you need the lamps to keep your temps up? If so, perhaps another method of heating would be better. Do you have a UTH?
I would eliminate the lamps altogether, put a UTH on one end set at 88-90 for the hotspot and, if you're ambient temps aren't high enough with that alone, you could add another UTH on the other end set at 80. Cover your screen top with wax paper or foil leaving a 1/2" strip along the back edge for ventilation. This should help hold in the humidity for you. This is what I do for my tank setup and my humidity is consistent at about 70%.
UTHs aren't expensive. I like the ultratherms from ReptileBasics.com. They are only about $20.
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