Well unfortunately, the harsh reality is that without a dimmer switch, rheostat, or thermostat, you aren't going to be able to control how hot those UTH's get. I've tried doing that with nothing but aluminum foil, but it wasn't Accurate and actually got too hot. These led to 2 of my snakes regurgitating, which was a big deal to me, but they were fine after 3 weeks.
Just honestly spend the money and get even just a dimmer switch from Lowe's or Home Depot and spend the $8-$12. Tubs are the best enclosures for Ball Pythons and the only thing you need to control the heat source consistently. Get a really good Accurite digital thermometer with a probe on it to measure the temperatures.
This is the best advice for the situation at hand. In any case a Thermostat is preferred, but sometimes a $80-$150 thermostat just doesn't fit into the financial priorities.