I think that just to be safe I'd probably put something under the pad so it's not directly touching the straw - then again that might defeat the purpose of the straw.
I'm only taking care of one kitty, and he's a stray, not feral, and somehow that makes a difference to me. I guess I just consider him my outdoor only cat, and I've spent quite a bit of money on him. 3 years ago, when I started taking care of him, I bought a small doghouse, not the igloo kind, just a regular kind of rectangle with a slanted roof. But there have been other cats (owned) in the neighborhood that have made their way onto my deck, and he's been spraying. He must have marked against or in his house, because he wouldn't go near it this year, even after I sprayed with Nature's Miracle. I then bought a small feral cat shelter, but the medium was really ridiculously small, and his bed didn't fit. So I ended up buying him an expensive cedar house. If he marks in it, I'm going to be really unhappy.