Honestly, for the trouble it is worth, you would be better off purchasing a nice 24" stainless range hood and relegating the microwave to another place in the kitchen. The other alternative is to custom cut some sort of stanless trim.

a 24in, cabinet leaves about 22.5in. space inside it.

"Ready to install 22inch OTR microwave in 24in cabinet" is your thread title.
So, if you have a 24" cabinet, you have just about the right amount of space.

They're installing a 22" OTR micro under the 24" wall cabinet, which presumably sits over their smaller 24" range.

Personally I can't imagine an issue with using wood fillers. They'll be up at the level of the microwave, and I presume you have wood cabinets on either side of your 24" range dropping to 18-19" off the counter surface.

What kind of range is it? There should be specs detailing the required space above the range before any combustible materials are allowed.

Given these trim pieces would only encroach over the range area by 1" on either side, I wouldn't worry about it - unless you go crazy with greasy flambe cooking on your 24" cooker ;)

Otherwise some simple metal trim painted white to match the micro would be relatively easy to make up using aluminum readily available at Home Depot or something - they should sell 1" x 1" aluminum stock (L shaped or full box) that you can cut easily with a hacksaw, sand the edges smooth and hit with some white Tremclad. Would cost you $10-$20 all in.

LG actually made 22"w OTR microwave/hoods a few years back. Sharp made undercabinet 24"w microwaves that attached to the upper cabinet, but no venting so not for over cooktops. May be worth trying to hunt down a used one.

GE doesn't have their Spacemaker II MWs anymore? There were almost 24"w and included undercabinet mounting kits (or had one as an option)

" not make wood fillers due to fire hazards" spayurdog just tell them to do it regardless of their concern. Underneath the wall cabinets, on the visible bottom plane, you could put some metal sheet / flashing to prevent any flames (from a stovetop fire) from getting into the wood that is everywhere in the vicinity of the microwave.

That is more fire prevention than the filler piece you are asking them to make.
Here you need a 1 inch vertical filler piece for the microwave to fit a space.
Horizontal sheet metal (how many square inches) under the floor of the wall cabinets is going to prevent fire from catching there, for a lot longer time, than a vertical 1in strip.

"a 24in, cabinet leaves about 22.5in. space inside it."

Thank you for pointing that out--I didn't even think of that!--will measure cabinet when I get home. If filler is needed I'll check with cabinet guys again--I still don't understand what the big deal is. The gas range is a 24" Delonghi (now Verona), and no, I don't do a lot of cooking.

I know how much everyone hates OTR microwaves, I do too, but my kitchen is SO small (I will post pictures when kitchen is finally finished since I had such a hard time finding examples of small kitchens with moderately priced, small-scale appliances. The dishwasher is the 18" GE Monogram and 24" Equator Conserv fridge.

thank you all for your input.

Good size dishwasher. 18" GE Monogram.

Interesting fridge. Equator Conserv fridge. I almost bought it.

Good luck with your vent ducting. I hope the vent gets used often and you write back, later, about the fan blower noise, and the general feeling you get (whether you like using it as an exhaust).

Sadly, this will be non-venting. I'm sure I'll live to regret it. Was trying to contain costs and my tiny kitchen has 2 doorways and 2 windows, so ventilation has never really been an issue for me. I've read that most ducted OTR microwaves don't do a very good job of exhausting fumes, anyway. I mainly want it OTR because there's basically nowhere else for a microwave to go (you'll see if I ever learn to post photos).