I have a 28, but no air conditioning. Was about to add it, then my wife decided we need a Catalina 315. Survey for that happens to be today.
But we're going to add air conditioning the same way we were going to add air on the 28. I'm going to get an under $160.00 home, window air conditioner. They're about 12" tall, and 16" wide. Going to set it right in the companionway, where it can drain into the cockpit while it keeps us cool. Yes, you have to step over the darned thing, but that's not so bad. Have to cut a couple special hatch boards and paint them gray. No big deal.
On the plus side, as we travel on Lake Michigan in the summer, no more hot, sweltering marinas in July and August. That's gets old real fast. Initial cost is way less than marine air. Waaaay less, and there's no need to put another hole in the bottom of the boat. Service will be a lot simpler, too.
Too bad the cockpit storage on a 28 is minimal. Still, if we were keeping our 28, I'd take the AC out of the dock box and put it in the cockpit anytime we were leaving on a trip. Maybe sew up a Sunbrella cover just to alleviate it's ugliness while it sits in the cockpit. The 315 has monstrous cockpit storage, and I think I'll sew up a sling that goes around the AC to retrieve it from the lazarette.
Compromise, compromise.