To help keep your humidity at proper level I first would suggest getting rid of the lamp. With a lamp you are going to have a real hard time with humidity.
You said you are having problems keeping your temps proper without out the lamp but are you taking surface temps or ambient temps? BP's do not bask so all they need a 88-92 degree surface (in the enclosure, on top of the heating pad) to provide them belly heat. In order to take a surface temperature you would need a thermometer with a probe (although these can be pretty inaccurate) or a better alternative is a IR thermometer which is designed for taking surface temps. Something like ( www.tempgun.com)
Getting rid of the lamp is really your best bet to help control your humidity. Some other things you can do is cover about half of the top screen with plastic instead of a towel to help keep in some of the humidity.
Also cypress substrate is the best at retaining moisture while suppressing mold and fungus.
Something like: www.serpenturf.com/