It looks like one of those trays you put under washing machine. I'm curious too to know what it is exactly and the measurements.
I don't know the measurements you need but here are some options to look for:
Martin's cages, cage pans
http://www.martinscages.com/products/supplies/pans.htm
You can also see if you can find dog kennel trays, made for cages at the pet stores like Petsmart or Petco. They are large sizes.
or
Trays used for under washing machines
It depends on the size of your home-made cage, but you may find something that you could use under it.