I wanted a tank topper like this one, except for mice.
I see advice that, for pet mice, the cage material should have maximum 3/8" (9mm) openings. Unfortunately, I haven't been able to find anyone who makes toppers like that, not even custom ones on Etsy or such. So... I decided to make my own.
It ended up being very labor-intensive. To go from 1/2" openings to 1/4" (6mm) openings (I couldn't find 3/8" cage material) means twice as many cuts, twice as many grinds, and twice as many hog rings. I started to understand why they're not offered — if sold, the labor costs would be enormous.
After I built mine, I discovered that one IS being sold commercially — the Ferplast Karat 80.