I’d build a 10x10 screen house for the cat. Large potted plants, something for the cat to lay on in the sun. He’ll be outside but contained.
..neighbor of mine had a cat stroller that used to sit outside with her in the summer months...looked like a mini pop camper for cats...
I’m going to second this reply by cripple. If you’re not going to keep the cat indoors, you can build it essentially a “run.” While I personally wouldn’t go to such lengths for the cat, you could build it out of pvc, and hardware cloth, much like a chicken run. It’d probably set you back $200, and you’d have to be sure to secure it, but really you have two options. A) Be responsible, and take appropriate precautions (this will cost time, and maybe money, and certainly effort on your part), which includes keeping the cat indoors, or building it a secure area. B) Be irresponsible, and let your cat out of your supervision/control, and then it’s none of your business what happens to the cat on your neighbors property, because you’ve abdicated ownership and responsibility of the animal. Really it boils down to that old choice: “right” and “wrong”.