Model: Homemade!
I wanted something big enough to comfortably house a dozen rats and safely separate boys and girls. I also did not want to hassle with a gazillion of different cages.
This 4'X3'X2' cage sits on a wooden table with roller-feet for easy moving.
It has three 'departments', each of these has an upstairs and downstairs.
The main cage has 1" wire. Since I have twice as many girls as boys the girls occupy the middle and right department and the boys are separate in the left department. The downstairs dep. are separated by wood, upstairs is separated by wire. The second story floor is 1/4" wire.
It's good for their feet because it's not too big but the 'poops' don't fall through and so I found it easiest to buy cheap terry-cloth shop towels (6 for $6) and they enjoy their royal blue "carpet" upstairs. I just pull it out, shake the poops into the trash and launder the towels.
Downstairs I was smarter and constructed three separate pans that hold the litter. It's a breeze to clean. I just pull the drawers out, toss the litter, put fresh one in. I use Aspen.
Accommodations: The boys have a balcony basket downstairs and I ought to get one for the girls, too. The girls have a house and a big piece of fake fur remnant, that they love.
Entertainment: Neither boys nor girls seem to like their rat wheel. I installed climbing rope all through the upstairs (1" Nylon rope) and they like to climb around on it. I heard sisal might cause allergies. I never had a problem with the nylon though.
The babies are born in a 20 gallon aquarium since the mom doesn't want the other rats around and the babies would easily escape later on through the 1" wire. I also have a second aquarium to separate boys and girls at age 5 weeks and an 'emergency'-converted rabbit cage.
When I don't have litters all my rats live in the same big cage and I love it!
If you are thinking about building one yourself, keep in mind:
It's going to take you longer than you think! It's going to cost you more money than you think! If you don't need anything as customized as my cage you might be better of just buying one! Email me for links to cage manufacturers.