Just a thought, and it may not work since you said it's an open bottom and you'd have to add a tray underneath or a solid floor... but when I needed more room for my quail in my rabbit hutch, I went down instead of out. It's about 2x4.5 and I had 11 quail in it, but recommended floor space per quail is 1 sq ft not including food/water. It was fine while they were just out of the brooder, then it got cramped.
I took a dog playpen that I already had with a built-in door panel and wrapped that around the legs, so it made a second condo-style cage sitting on the ground. I've got it all backed against my house so I just added protection on one more side, and magically I doubled my space. Currently I've got 4 females in the top and 1 male and 3 females in the bottom, and they're doing great. They're covered because of the roof, have the side as protection, and they're within a fenced off area inside my also-fenced area for my dogs, so if something wants to get them they've got to go through 2 fences, 3 dogs, and the cage to do so. I think something similar could be done with hardwire mesh, just attaching it to the legs and making an access door.