Run down to your local garden supply store and pick up some seed starting trays. They're usually styrene plastic, about 9 x 19 inches long.
The only problem is that they're pretty thin, so you need to nest two of them together so they'll carry 2 or 3 liters of liquid without breaking up when you lift them.
For about $2 apiece, they make simple, readily available panoramic format trays. Just make sure you don't get the kind with drain holes punched in the bottom....