I used plywood covered with carpet remanents for extra flooring in my
Midwest but I've seen people use something that looks like Masonite or
counter top too.  You have to measure the inside diameter of the cage, then
MEASURE THE DOORS.  Figure out how many pieces will fit across (I did my
pieces the short way) that will also fit through the door.  Get the pieces
cut at the store or if you can cut your self.  Get some hooks, drill,
install.  Also allow for ramps, i.e. your floors will not be completely
across or if you can, cut holes out for the ramps (this can make attaching
them hard) make sure you experiment with the ramps on the different
configurations first - they don't work in all directions, before you buy
and cut your material.
 
As for flooring, I took 3/8" plywood bigger than the diameter of the cage,
stapled linoleum to it and just set the cage on it.  It doesn't roll but I
don't need it too right now.
 
Currently I actually have 2 4-5 week old foster kittens from the H.S.
living in it.  One of which likes to sleep in the top hammock!
 
Mary, Booker the dog, Boris and Giesela and the two foster kitties
[Posted in FML issue 2347]