Your best bet is to use a glass lid made for a 10g tank. They cost around $20 on avg. you can drill holes in the plastic spacer to allow a small amount of ventilation and that should keep issues like mold and mites at bay. Expect some white mold at the onset of the tank if you have natural wood in there, it's normal and goes away on it's own in a few weeks.

Opening the lid daily to feed will give enough air exchange for the frogs.