2 feet of 11X 12 inch flexwatt, cut down to cover just about 1/3 of the bottom of the tank.
Reptile basics for the flexwatt.
Wire to a lamp cord and plug it into a thermostat.

Hydrofarm or repti-temp 500r thermostat or if you want better quality step up to a johnson controls A419 thermostat.

Cover 1/2 or more of the lid to help trap the heat in longer.