Blocking off the top is a rough way to try and retain the humidity since it hacks the airflow, but if you must cover it, try covering as little as you can with wet towels (towels still breathe to a degree whereas plastic and wood does not).
Some other ways of increasing humidity in a tank are:
*place a large dish of water over the hot spot (assuming you are using a UTH - this method isnt as effective with lights since lights dry the air out)
*use moist hides (these can be as simple as a tupperware container w/ a hole cut out and filled with damp moss)
*live plants added to the vivarium increase humidity to a degree, but should you choose to use these make sure you "quarantine" the plant for 30 days - remove plants from soil, rinse the plants itself (not roots) in soapy water then rinse well and repot the plants, then daily rinse the plant for the remainder of the quarantine.
*misters/foggers work quite well, and waterfalls can also add humidity