Vents, HELP!!!?

Question:i have wooden vents(you dont screw them down) in my home and my cats like to stick their paw in them and pull them up. I have to put them back down all the time!! i just recently got a new kitten and im afraid it will get down in there. What can i put under the vents so that the kitten cant do that? i used to have those plastic things you put in them but i cannot find those. can you buy them anywhere? what are they called?

Answers:
Try buying some heavy screen at home depot. Strong enough so that the cats can't tear it. Cut them so that you have some margin to staple the screen into the surrounding casing since the vents do not screw down. Measure the vents and maybe they'll cut it for you at home depot to fit under the vent. The air will still pass through the screen of course.