For that matter, have you tried wrapping any kind of insulation around the tubs? Styrofoam is great at retaining heat. If insulating a tank works, why not a tub?

You could also try an additional heat mat or UTH mounted to the side. It should still be regulated obviously, but a dimmer should be fine for starters. It won't get as hot as on the bottom because it won't have substrate over it to keep heat in, and the snake can't just sit on it and insulate it with its body and get it up to dangerous temps.

You could also try heat lamps pointed at the lids or sides. This is very crude, but even a normal incandescent bulb will make a fair amount of heat and if the lid is opaque and the sides are covered, it should just warm things up without the light bothering the snake. So you could point a lamp directly at the lid and get some heat that way. A CHE is probably better though.

You probably don't want to spend the money at this point, but I use a RHP on my 20L to get the temps up and it works great even though the room temp gets down to 55 degrees in the winter (during the day when no one's home and overnight when we're under the covers). It does always seem to me that a lot of the advice for heating snake cages is great for people whose houses are 15 degrees warmer than mine! Yes, I could also raise the thermostat... but heating the entire house (or even just the entire room via a 1,000+ watt space heater) because two or three cubic feet of it need to be warmer does not strike me as efficient!