If you have a mesh top I would suggest trying aluminium foil! I covered 3/4 of my top woth 2 layers of aluminium foil to help with humidity.

After a day my humidity levels averaged out to a constant 52-63% depending on the time of day and wether or not it's hot out.

The temps seem to have stabilized as well. If you haven't tried foil on the top I would suggest giving it a shot.