I usually use 2 sheets of newspaper, you might try that. Also, is the UTH on a thermostat? If so, what have you set the thermostat temps to?
I haven't found that adding more ventilation holes reduces the heat. If you'll go back earlier in the thread and look closely at the pictures I posted, you'll see two rows of holes along the upper half of the tub.
However, you could try a smaller water bowl, or move it more to the cool side.