If you just prop lattice up against the screen frame, be aware that some screens will actually pop out if pushed on too hard (many are designed to so to make escaping fire easier). Be sure yours are nice and secure before you trust it too much. Definitely don't just lean it against the screen itself and trust it, if he tries at all, it will pop the whole screen right out of it's frame, and out goes screen, lattice, cat, and all. If you have deep window sills/frames, another pre-made option is a baby gate, just be sure it's got holes small enough to hold in a cat.
Oh, and yes, tom cat pee stinks to high heaven. It should be MUCH better in another week. Sometimes the actual CAT reeks after a neuter, and requires a post-neuter bath. Whitey had to have one, he was icky! My vet says after one week, it's OK to bathe the cat (you don't want to do it before then since the neuter leaves an open wound and bath water can carry nasty gak into it). I usually totally dislike clumping litters, but that would probably help a lot, if you could remove the PEE from the box regularly by scooping. Just be sure it doesn't stick to his neuter sore. I like the Feline Fresh brand scoopable, but other good, safe brands are World's Best cat Litter, and SWheat Scoop. Phaewryn http://ucat.us/domesticcatlinks.html Special Needs Cat Resources
