OK, I think those side panels are called filler kits. But what do you think about putting a 30" microwave/vent over a 36" range? Not in terms of aesthetics, with the filler kits, but would the vent work or would it be useless?

Fire hazard with wood fillers. Stainless filler would not be cheap. An OTR is better than nothing, but not by much. A 30 over a 36 goes in the category of hopeless optimism to be dashed by a smoky and greasy reality.

They do make metal filler kits as an accessory to adapt a 30" wide microwave to a 36" space. They don't really improve the capture area like a hood canopy that is wider than the actual intakes for the vent.

And the filler kits are $100-$250, so that could be taken into account toward the purchase of a 36" microwave. However, I looked and could not find an actual 36" wide microwave, 30" is the widest I could find.

So your old OTR didn't vent outside? Maybe you could find a recirculating hood to fit the space, and put filler above it to bring it down closer to the range.

One of the goals of my kitchen remodel was to get rid of the OTR MW and put in a vent! If you're on an outside wall, it isn't that expensive.