I haven't seen any other cages like the one I made, so thought I would post it here in case it might give anyone interested some different ideas. My rats looooove it and so do I.
I bought a metal shelving unit from Lowe's (edsal 72"H x 48"W x 24"D Steel Freestanding Shelving Unit, Item #: 101933 | Model #: CR4824). It's 60" high but I only enclosed 42" of height, with a shelf above and below.
The back and the floor liner on the bottom section is FRP (plastic paneling). The sides and doors are covered with 1/4" coated mesh. The door frames were easy to make from do-it-yourself window screen frames. A cabinet magnet at the top and bottom of each door holds them shut very securely. The middle and top shelves are made from wire closet shelving, and lined with choroplast covered with fleece.
The mesh sides are attached to the inside. It was more trouble but I thought it would be safer, to make sure they could never get between the mesh and the shelving frame:
Luckliy I found hinges with the same hole spacing as the frame, at Walmart:
Closeup up how FRP panel is attached to the back:
The leftover piece of FRP after cutting it to size was used to cover the bottom shelf, which is one of the wooden shelves that came with the unit (painted, but never gets wet as it is protected). It's very easy to clean, so I don't cover it with fleece. If you buy this stuff, let it air out outside for a long time as it has a smell to it when new. But I researched it and found that the material is not toxic. Still, I wouldn't want them to chew it so I made sure no edge is exposed by covering edges with plastic panelling trim.
Hope this gives you DIY-ers some ideas!