I don't think you need to add any strips of wood if you are using a trim kit. but you can double check on that.

if you are not using a trim kit, you can add two blocks of wood on the side to "narrow" the opening to the appropriate size.

Thanks, Noopd. I was under the impression that the trim kit just covered up what ever makeshift method was used to make the space smaller. I will look into this. Would really make my life easier if it does it all.

I had to install a 24" Gaggenau combi steam oven in a 27" cutout after the 27" was discontinued. It was replaced under warranty.

The installer used strips of wood behind the oven at the sides to mount the oven. I was left with gaps on each side of the oven.

Gagg did not make a trim kit, so I had a custom trim kit made by a company called Microtrim.