you can add a basking lamp and a timer. that should bring you up to the correct temp. also if you have a screen top try and cover some off it up with a towel or something i used the ziplock press and seal
What kind of thermometer are you using to gauge temperatures? I agree if you are using glass/screen lid cover with the glad stuff or clear ConTac paper - this will both hold in the humidity as well as allow the warm air to be retained and elevate temps.