Did you have the UTH on while you were checking it?
I keep my humidity up by using larger UTH's that cover about 1/2 of my tubs floor space. This allows me to place the water bowls over the UTH so it evaporates faster.
When you go to Wal-Mart, check in the luggage aisle and see if they have any luggage straps. I like them better than bungee cords because they are flat, and adjustable. They don't have to be super tight. Just tight enough to keep the lid from raising.