Consider this: If you are using it under the cage, maybe a PVC board could be bought and bent into a "v" shape to allow for drainage out, or make any shape you want out of wood and apply cheap linoleum over the surface.
A little more information on how you plan to use this tray (i.e. inside or underneath, drainage, planter, etc.) might get you some more replies.
Also, if you go out to a hardware store you can usually find water trays for potted plants. I have a a large round and I know most plages have square shaped pots to buy. You might get lucky and find something, but you would probably have to buy 4 1'x1' and use sealer to glue them together.