If you can borrow a propane torch its quicker to make your own,
Purchase 1/8 inch copper tubing , heavier if your cat is really big.
Bend it to fit the inside of the port nice and snug.
Place it on a flat surface and lightly hammer it flat.
It will grow from hammering so trim again.Clean the ring with scotch brite or fine sand paper.
Solder copper screen from the hardware store with the borrowed torch.
Radio solder usually has flux in it to ease the job
Trim off the excess screening and lightly file the ring to remove stray screen.
Done,,,, Creating a dozen at a time takes little more effort or time
Bigger ports like a 12 or 14 round will use thicker copper tubing .