I was considering this as one solution but did not know if that would look like patch work, your thoughts?

I don't think it will look like patchwork if it is one piece. If it matches the cabinets, the eye will not be drawn to it, and it will not even be noticed.

The correct way to do this is would have been for your fabricator to have the plywood go all the way across and not just the strip behind the mitre edge. We use marine grade plywood & cover the entire underside wherever the area has an overhang specifically to avoid this "unfinished" view.

That is actually a fairly neat finish as most just cut plywood to rough shape and lay it on top. Agree the above would help but you would still have to deal with the plywood.

Leaving unfinished is fairly common practice and at 36"? I am not sure without bending down how you are seeing this?

I would get paint to match your cabinet color, tape off the marble and paint the exposed wood. It would all blend in. Again, I would like to see a photo from the door at eye level.

I think it looks fine and there is no shoddy work obviously visible here. Hey, sometimes we have to stick up for the contractor. Looks great to me?

HI -- No one said the contractor did anything wrong or shoddy . Just get the cabinet company to install a thin gable panel that matches your cabinets . Or you could put a thin piece of Masonite under and stain or paint it a dark color as close to the wood color . Or a panel of galvanized metal or stainless steel or a Formica laminate material can be applied . This actually should have been for the kitchen designer to remedy after bring it to their attention and not the contractor .