72 is not that bad... but if you want to bring it up 3-8 degrees higher which is also ok..the only thing that would bring it up a small amount like that would be a heat pad on the side. Another bulb may bring it up to high... and a ceremic heater would put out to much..even a 40 watt for that size of a tank. So I would start with a heating pad...but keep in mind 72 isnt that bad either for a whites tree frog. It also depends where you keep your thermostat in your tank. may be 72 in one area and 78 or 65 in another.... you know? Wise move on the upgrade of the tank size!

72 is a great evening temp btw...Those frogs are ok all the way down to 65. Im no expert but Im not sure where this 85 degree basking spot came from for a whites tree frog. The temperature in their local climate doesnt even get that high until the summer. The only frog I know that basks is the Giant Bicolor Tree Frog.