Couple of thoughts since we went from a Journey (same cabin as Voyager) to a Sailfish, both with AC we rigged on the 258 then installed on the 282.
The 248/258 has fairly long berths, longer than the Sailfish. My wife and I are both pretty short. We could place one of the cheapo 5,000 BTU units mentioned above at the foot of one of the bunks. We found something similar to the shape of our window (a car air intake?) and duct-taped the exhaust up to it.
For power, we put a 110v inlet just below the helm that has a pigtail that resides in the webbed storage bin inside the cabin when not in use. We had to carry a 100', heavy gauge extension cord to make sure we could get 110 power in a marina.
That setup worked well enough but that little AC sure feels much more in the way in the cabin of a 248/258.
A better and much more expensive option is to install something like a real marine grade 6500 btu AC. I used a Mermaid brand in my Sailfish and so far I'm very happy with them.
But - where to put it? It might fit under a berth. Or, as suggested, maybe under the sink, but I think that will be tight. Or, if your boat doesn't have the second fuel tank forward, you may be able to fit it in that space. That may work well for a good intake vent from the cabin as well as for placing a thru-hull. You'll also need either a shower type sump pump to pump out the condensate or a place for it to drain to a bilge pump. Perhaps you can run a sump pump to your sink's overboard drain.
In any event, your boat will need to add shore power for the installed marine AC, another expense, if you don't already have it.
Tx, Rob