I have a 3-1/2 x 12" vent, fed by a 3-1/2 inch x 12 inch wall vent pipe, which is tilted up so any water that blows in will flow back out again.
I had a filter to keep bugs and bees out, but it restricted airfow too much and the A/C unit shut off. I'll either put a screen behind it like Miriam, or make a cover for whne it is not in use, which is almost always.