First off YES you never put tape in the enclosure. EVER!!!
Second, you can tape the probe to the bottom of the UTH but you will need to make more adjustments.
The UTH is for a hot spot and NOT to raise ambient temperatures.
At this point you need to clear all the substrate from over the UTH and use a no contact or temp gun to read the surface temperature.
Now you will adjust your stat until you have the temo you want.