Another option (no laughing) is a golf umbrella, modified to act as a lid. Merely open the bumpershoot, glue the locking ring in place with gorilla glue, and the cut off the handle. Grule a cap, or wrap some tape around the handle stump.

Viola! Instant frog enclosure cover.

You can hold it in place with some adhesive Velcro strips to prevent escapes. And, if you are handy with a needle and thread, you could even sew in some widow screening as viewing ports.