I had a new double-glazed panel for a conservatory door made with a round hole in it (I think you do round in glass/PVC, goes without saying to buy the cat flap and check size etc first). It’s worked a treat. Was expensive (£200 I think but that was for a full panel, you’ll only need half) but much better than a stinky litter tray in the house.
Make sure you keep the old panel as when poor kitty moves on you can put it back in and not have a great draft through your door. Wish I’d thought of this before.
Lee Glass were excellent, not surw how national they are though http://www.leeglass.com/