I've got something similer, but I put my plants on a big tray. I then put one of those egg carten grid things that they use for florescent lights into the tray so that the plants are not sitting on water. I just let the excess water evaporate from the tray to add humidity for my plants. Hope this helps. =)

If your using regular nursery flats you could line them with plastic sheeting.

Invest in some sturdy trays, such as Permanest. (I got a good deal at Dan's Garden Shop online.) They make nice ones about 11x22 inches, the same size as a flat, and you can fit 4 on a shelf with a little squeezing. After I killed a couple shop lights by watering them, I invested in the trays and I'm glad I did. Don't skimp and try to use the cheapie black nursery trays - they will leak very quickly.