I use the 22" X 48" trays from Housermans. They fit nicely on my racks and I can use a tube (included with trays) to drain the excess water or plug the drain to use as humidity trays. They are the same trays which come with their light carts but are available seperatly also. They work great.
I've purchased many, but I've also made a bunch of mine. All you need is some deep trays and a sheet of eggcrate, purchased at Home Depot, Menards, or etc. I score it with a box cutter on both sides and then it breaks easily on the scored line.
We make our own using whatever shallow plastic pans we find to fit the spot we want then set egg crate light diffuser on top. Put a couple upside-down pots in to keep the egg crate from bowing and voila. DIY is great. We have some nice 2'x2' square x 2.5" deep plastic pans that we don't have in the catalog because they're probably expensive to ship??? If you're not in any hurry, my parents live in Lexington, we could toss them out the window in Louisville next time we zip through. USPS works too. Contact me if you're interested in some.