This is about the cheapest/easiest cage idea I've used.
What I did was take a 50 gallon storage tote and mounted it on a sealed-wood face frame. For your needs I would seal the back of the frame with rubberized contact paper.
The box is attached with polyurethane caulk and several sheet metal screws. For your cages I would use silicone and the screws.
The door is a bit expensive but there are ways to make it cheaper. Probably double sliding, 1/8" glass doors would be my choice.
Sorry for the condition of the cage in the picture. A friend of mine had a Corucia zebrata born and needed an emergency cage for the mother and newborn. They trampled the cage pretty good with that first night of exploration.
I'll post a second picture in another reply.
