I would just make slits. A whole in a top of a tray is going to let too much water in, I think.

Do we have gremlins here or what - first a double post and I know I answered one, now its not here ;)

But basically what I said was the same as above, if wider than a milk jug, you'll find several slits across the top will work better. The only thing I'd added was make sure the slits are slightly open, whether that means rocking your knife or tool back and forth some, or actually cutting slivers out with the lid inverted on a board before covering your container with it.

I do little x's with my knife as opposed to long slits. As morz says, you need to make sure the slit is open and I find the x's make that happen.