Since you just have one bp for now i would pick up a digital thermometer and that dimmer switch form Home Depot/Lowes. If you can find an accurite temp unit( about 10 bucks) at Home depot/lowes that would be best as it provides humidity and two temp readings at the same time. You can place it at one end and run the probe to the warm side.

If your bp is still young a 20gal might be a little big so just fill it with alot of stuff to take up the space or you can make a cardboard divider til it grows into it.