In order for a tub or rack setup to work with just a uth, you have to keep the room ambient temp up, 75 or higher. UTH will make a hot spot but isn't going to do much for warming the tub air up. Right now work on temps and don't worry about humidity, bps do not need super high humidity, if your snake goes into shed you can mist the tub daily and he will be just fine. I run a small room humidifier in my reptile room and have no problems.

If raising temps isn't an option you need to think about getting a plastic cage you can add a radiant heat panel to or a tank with some lamps to get your warm side up. Thermostats are needed to control any source of heat, I use herpstats. If your uth is not on one please unplug it, warm to the touch means it's quite hot since it must be over your body temp. They can spike in temp and run unevenly, burns are not worth risking.