Maybe the plexiglass is your problem. Have you tried the metal mesh screen tops?

I remember one time I was keeping 2 rats in a tall rubbermaid. I didn't have a lid, so I drilled a LOT of holes in the plastic lid I got with it. I left to Home Depot to pick up supplies to make a wire mesh lid. A few hours later when I came back, the sides of the tub were completely soaked, and when I opened the lid, a rush of stench slapped me right in the face.

I made the wire mesh top and put it on top of the tub and never had that humidity build up again.

It didn't matter how many holes I would have drilled into that top, there was still too much of the lid and it kept the humidity (and the smell) inside...nothing could escape.

Your best bet is to either purchase a screen top or make your own from Home Depot

I hope this helps