Also, do yourself and your snake a favor and take that analog thermometer/hygrometer back to the pet store. It will not accurately measure anything. Go to Walmart and pick up a reliable digital thermometer/hygrometer combo unit with probe. It will save you alot of heartache.

Also, if you want a cheaper, natural substrate, why not try aspen? Covering up that screen top is your main concern. It will help keep in humidity if you do that. Using saran-wrap, glad Press N' Seal, plexiglass or ConTac Paper. Cover up the entire lid except for 4 small triangles at each corner. You can keep good humidity with that covering your lid even if you had paper towels as substrate.

And please, for the sake of your snake's wellbeing and LIFE, do NOT use a hot rock. They can and will burn your snake. Snakes and all reptiles do not have heat sensors like we do. They are very slow to detect heat and will not move, even if they are being burned. They just don't know they are hurting themselves. That's why it is OUR job as an owner to prevent this from happening. Using an undertank heater will prevent this as well as accurately measuring with a digital thermometer.

All of this will be cheaper in the long run because you won't be having to take your snake to the vet because of massive burns all along his belly and sides. Vet bills are not cheap, and if burns are not treated by a vet, a massive infection could easily take your snake away from you.

I really recommend that you do not use a hot rock. They are not an adequate means of heating an enclosure and will do extreme damage to your lovely snake. I would use a combination of an undertank heater(flexwatt is very inexpensive but needs to be used with a dimmer or thermostat as does any other undertank or overhead heat source) on a thermostat as well as a lamp on a dimmer to maintain correct floor and ambient temps.

Cover up that screen top with something other than a damp towel(EEWW) and get proper heating, and you will be set.

That's just my $0.02 as a medical professional though...