Well what you can do, this might not be the prettiest thing. But go ahead and get yourself some black foam board or even reflective duct sheeting from Home Depot and put it on the sides and back of the tank to keep in heat.

Go ahead and spend some money on a good thermostat or just get a nice heat pad ( I always recommend UltraTherm, go to reptilebasics.com) and go to Lowes and get a dimmer switch for a lamp. The thermostat is going to be your best friend, but in a money crunch mess around with a dimmer switch for a couple days to get it dialed in and make sure you have plenty of good thermometers to measure it with.

If you can get the cold side to no lower than 75, and get the hot spot somewhere 89-94, then you'll be fine. Even if the hot spot is 86 or 85, chances are the ground is going to be warmer and probably perfect temps, just double check everything, make sure the heat pad isn't going crazy and you'll be fine.