That's what I did with my Exo Terra. My Whites Tree Frog is about 2 1/2 inches so not nearly as small as your guys.

I just took some toilet paper/towel and stuffed it back there for now. He hasn't tried to get back there since, nor the crickets I feed him. I don't know if the duct tape would work or is safe, but the toilet paper/towel has worked great for me. Just make sure to pack it in tight so they can't squeeze through, and make sure to check none have gotten back there if you can't find one.

Thanks,
Alyssa