The uth comes with spacers, you put 1 in each corner of the tank. That will provide adequate airflow. Cover 2/3s of the top of the tank. That will keep your temps up on the cool side plus you will hold humidity better.
Please buy a t stat, I know they are expensive but they are worth every penny. I used a dimmer and had temp fluctuations between 92 to 100 degrees.

The t stat is worth it's weight in gold. You can use flex watt for heat and its way cheaper then a uth. Like $4 for a 1 ft long piece piece. I know a medium uth is $17 or more.

Why dont you switch to a sterilite tub? It is super cheap, easier to clean, and easier to manage temps/humidity.