Check the temperatures in several places, first. What wattage is your CHE?

If your tank is too cold, you have two choices: Insulate it to trap more heat, or get more wattage to heat it with.
To insulate your tank, cover three sides with a material such as foam insulation, foam core, cardboard, etc. There are lots of choices. That will help some. In general, insulating the tank will probably also even out the temperature a bit so there's a smaller difference between the warm side and the cool side. Whether or not this is a good thing depends on what the difference is right now.

To get more wattage, either switch to a higher-wattach CHE (make sure the lamp fixture is rated for whatever wattage bulb you put in it!!) or add an additional heat source. The additional heat source could be a UTH, a lamp, a RHP, etc. A UTH isn't the greatest for raising the ambient temperature because most of the heat is blocked by the substrate. And since it is insulated by the substrate, it needs to be regulated by a thermostat and probably will rarely ever run at its full power because otherwise it could get too hot and burn your snake (again, this is because it's insulated by substrate). And a UTH is a pretty low-power device to begin with.

If you have trouble with humidity and you have blocked off part of the screen top to solve that, you probably don't want a second lamp fixture because you'll have to un-block screen to let it shine through. So that leaves either a higher-wattage CHE or a lamp fixture intended for two bulbs or a radiant heat panel. Most people usually think of radiant heat panels as going together with PVC cages, but it's not hard to mount one to a screen top on a tank. You just have to cut a piece of wood that reaches past the edges of the screen to mount it with. The wood goes on the outside, and the RHP goes on the inside. Screws go through the screen into the wood.

If you get a second heat source, you can put it on either side, depending on what the gradient looks like. You'll have to try it in a couple of places and see what you get. Don't worry, your snake will be fine with the temperature fluctuations during that trial and error period as long as it's within safe ranges. Too hot is way more dangerous much more quickly than a bit too cool.

It goes without saying that any heat source you use needs to have a thermostat.