Can you use under the tank heaters on the SIDE of the viv?

With an 80 watt radiant heat panel at full power, the warm side of my tank is still about 3 or so degrees lower than I need it to be. (It's because this tank is 2 feet tall, taller than my usual ones) It's more noticable inside it's warm side hide-box which is a thick cardboard box. I think maybe the box blocks the heat from reaching down inside there. It's about 5 degrees too cool in there.

How do you think I should work about correcting this? I only need a few degrees difference...

I was thinking about using an uth, but I'm not used to those so could you recommend a brand and wattage?

I use stacked critter condos so they can't go underneath without being completely squished, I think they need some airflow...I could put lil' pegs or something between them to raise one tank up a bit (how high would I need?) but I want to look at other options too.
How about if it was stuck on the side of the enclosure, would that work? Could you tell me a bit about it, cons/pros? The side is made from epoxy coated wood about an inch thick, the heat might not be able to get through...Is my first idea, raising the tank up and putting it underneath like normal, better? or would I run into the same problem?

Otherwise is there any other ideas to raise the temp in a critter condo just a couple of degrees? Would raising my room temp by that much have the same effect inside the tank? If it's that easy I'll get myself a space heater.

THANK YOU FOR YOUR ADVICE!