Many times the best fix in these types of situations is to contrast the repair. Maybe a row of decorative tile or solid surface? Better to look like you planned it that way instead of "almost got the stain matched".

I appreciate your help because we never would have thought of these idea on our own.

You could also look at separate appliances (a single oven and a microwave). That might fill up more of the vertical space in the existing cutout thus minimizing the size of the wood rail the cabinet maker would add between the two. This approach has the added benefit of letting you the appliances separately if they don't fail at the same time in the future.

A simple filler and an overlay filler, finished to match (the hard part) would be the one way.

A piece of stainless would be cheaper and easier, more than likely.

We've thought about a shelf with two separate appliances because we'd definitely be happier being able to choose appliance separately. Having taken the face plate off the front of the current unit so that we could look inside the opening, we now know that in addition to building a shelf, we'd need to put a liner piece of wood over one side of the cavity because what is there is not finish quality material, and we'd need another piece of liner in order to bring the back side forward because now it is too deep for a simple shelf for a microwave to sit on. Is this terribly difficult to do?

Taylor's cabinets, can you please explain the filler and overlay filler to me? I do know what you mean about the stain being difficult to match. We just visited a kitchen showroom, and I saw several cabinets where they had to slightly alter the size to insert the new display oven, and despite the fact that the cabinetmaker had done a good job, I could still see the patch.

Thanks again.

"... despite the fact that the cabinetmaker had done a good job, I could still see the patch."

This is what you want to avoid. A contrasting fix and a "we planned it that way" attitude is much better, especially at eye level.

A filler is just that, a piece to fill a hole. The overlay is typically a filler with a deco edge, or a fancy filler.

Getting all this to matching the existing finish is the hard part. Even if the you can get a custom match stain, how the parts are sanded, and how the finish is applied will effect how the end product looks.

I would welcome suggestions on who to contact regards a stainless filler piece

I'm having the same issue and have found another thread suggesting a custom trim fabricator called Microtrim. https://www.yelp.com/biz/micro-trim-santa-ana-2

If you came up with a different solution, please share! Thanks.