Here are some thoughts. Ditch the mercury vapor bulb and get a lower wattage flood bulb from a hardware store. Here is an example for comparison. A 35 Watt flood bulb (PAR 30) from 10 in away give me a 115F basking spot for my Tegu. Get a fluorescent fixture (T5HO) to provide UVB, that will help with killing the humidity. In addition I was going to suggest topsoil until I re read your post. Quite honestly you could dig up some dirt from your backyard and mix that in, it is pretty common practice in the large lizard community. If you are uncomfortable with that look for some DG (decomposed granite) at a hardware store and mix that in with your substrate that should help give it some moisture retention, plus when you do get topsoil you can just add it to what you already have.