What to use to block of a hole in a rat cage?
Get a new cage. In the mean time, use plywood.
Something they can't chew through.
Metal wire .. rats can chew through plywood in a single nite if inclined to do so. They are very tenacious.
You can use a piece of window screening.
Go outside and get a rock...
A pile of books? Maybe putting duck tape on both the inside and outside of the cage. Maybe you could somehow attach a rat/hamster ball to it if its one they could get through