I've gotten aluminum laser cut for less than I could buy small amounts of material for. Usually they just throw my part in with production of other pieces being cut from the same sheet. Maybe the places you've asked don't want to deal with the hassle, but it doesn't actually cost that much to do. If you were friendly with someone, $50 could potentially get you a laser cut aluminum surrounding, like a picture frame style, rectangle within a rectangle.
Along the same metal trim idea, a sheet metal shop could shear an aluminum rectangle to match your outer dimensions, then you cut the inner rectangular window with a jigsaw. I mention aluminum because its so much easier to work with than stainless. Alloys 3003 and 5052 are common for sheet and are decent for not corroding too much. Might want to spray with clear coat though. And when cutting, be sure to keep the protective plastic on it (if existing) or cover with masking tape. If it doesn't already have a brushed finish, use sandpaper or wire brush.
Otherwise, wood trim should be relatively simple. If you have trouble matching the finish, it sounds like painting black shouldn't look bad if its inside the original enclosure. I think you're overthinking this a bit. I gave you some complex solutions regarding the metal, but if this is all a recent replacement of the microwave, I don't know how much you'll notice in another couple of weeks.
