I have the same problem. My critter room has 3 outside walls. First off...pay a bit more for heat each month!
Use a UTH and an overhead heat light (nocturnal red, probably). Hook these up to timers and a thermostat so you can have either one mechanism on or both on, depending on season and time of day/night. Make sure you have at least 2 thermometers in each set-up (i'm sure you do already). The heat light will heat the substrate below and any basking areas, so it alone can provide sufficient belly heat (terracotta absorbs heat nicely). I've never had digestion problems. Natural substrates hold more heat nicely...but they also hold more humidity, so watch that.
I don't have any arboreals so I use long set-ups. They provide better heat gradients. I used to have snakes and in my experience, Balls and Corns are really hardy -- I've seen them survive fine with no heat gradients...but I really don't recommend keeping them at constant temps. They prefer gradients. My leos don't seem to care about gradients (though they do have warm/cool availlable) and they like to bask right under the light at night. Finally, some of my sub-tropical/tropicals get REALLY pissed off if the temps don't drop to the low 70's at night! It totally depends on the species.
Alternatively, you could outfit the set-ups with Flexwatt and attach them to thermostats (dif. ones for different critters). I found flexwatt really annoying for trying to maintain my leo set-ups but great for snakes. Now I'm just using UTHs and lighting fixtures because I don't want rack systems -- I like to be able to see my animals.
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Too many Leos
1.0 feline "Spot"
0.1 canine "Tika"