Hello and welcome to FF Jennie. First thing to do is read this article: Frog Forum - Pacman and Horned Frogs - Ceratophrys - Care and Breeding. The fact that frogs dig down to escape heat is the reason under tanks mats are not a good choice for frogs. You can place them on the side; but on that position are quite inefficient with heat escaping all around them. You can use several options or combine various heat sources to get 82-84F daytime temp that can go down a few degrees at night. How long is your frog from snout to vent (SVL)? Unless a big female you can keep in a 10G and that would reduce heating needs almost in half.
The maximum wattage you can use will depend on the light fixture with plastic ones tolerating less than metal domes. If you check inside it there should be a decal with a warning or if you know brand please post and well go from there. If a dome; best solution is to use a ceramic heat emitter (for a 20G think a 100-150W will do) with either a dimmable dome top or the regular dome connected to a manual rheostat or automatic thermostat controller. Other option to down facing emitters are red colored bulbs that can be used 24 hours on.
To minimize heat loss get some aluminum foil and cover your whole screen top. Black electric tape works well for taping to corners. Then place light fixture on it and using a sharpie, mark around it's edges. When you cut hole open with scissors make it 1 in. larger around whole circumference to provide ventilation. That should help lots to retain heat and using the dimmer or controller you can dial the correct heat level in. Another option is heat tape fixed to back tank wall or sides. Do be aware than using either heat tape or a powerful heat mat on side glass, could crack it, if cold water splashes on it.
Dubias are fine food, watch they do not bury down before frog eats them. The best are night crawlers and you can get the not dyed kind at Walmart or local bait shops. For an adult frog can feed them whole. Either food should be dusted with CA/D3 and vitamin supplements on different days, here are couple schedules to use: http://www.frogforum.net/food-feeder...schedules.html.
Change it's bath water daily with dechlorinated tap (Seachem Prime works great), provide a hide or cork piece to burrow under and a silk plant or similar and all should be well. Good luck!