Could you do something to lift the trays/pots up and create a moat, like a glass jar in a takeaway container full of water? If it is a square tray you could do one on each corner, then you just need to make sure you top up and tip out to remove mosquito larvae periodically. Add a couple of floating sticks or cork and you have a safe water source for bees too.

For seedlings in the ground I've had some success cutting soft drink containers to create little circular fences around them. They need to be secured with a peg (a piece of wire coathanger bent over works well) otherwise they get scratched out by birds (or removed by mischievous puppies "helping" in the garden!).