You need a cabinet maker, or if you can figure out who manufactured the cabinets you have, you might be able to order a full pantry pull out to match your existing. Something like this:

You probably also have a gap at the top . I'd try to right size the fridge. By the time you horse around , the appearance will be far better if it fits. Or get the cabinet maker for all sides.........including above. Forty two inches means a taller fridge. Sound like the town house was designed with some luxury in mind.

Another solution might be well made custom stainless panels made locally to fill the gaps. All three sides ( affixed to plywood as a "frame" around a 36 inch standard size stainless.)

I assume/hope you will keep to the counter depth variety if that is what you have.............

Yes, we will use counter depth. The opening is actually only 72" high, so it's an odd fit even if we did have the budget for a really high end fridge.