Belly heat. Flexwatt, heat tape, or a heat mat, any of which you choose requires a thermostat (something that controls the heat on them), I think Amazon has a sale on a thermostat for reptiles for like 20$ on one that's normally 50.
Lights and BPs don't mix. If they are heating the enclosure, they're taking your humidity out. If you have heat sources inside the enclosure, you have a cooked reptile.