There might be filler pieces available depending on the material and color used.

Yes, thank you. I believe there are. I was also thinking couple of shelves. Easy to build but would catch dirt and grease. Pull out spice racks would be okay as I have them beside the stove. But harder to build (this will be diy) and would hate to pull them out if the size of the microwave changes next time. Appliances these days only last 5-8 years unless you buy super high end which we can not afford. Looking for some innovative ideas. I am installing a new quartz counter top and hopefully a glass tile backsplash. The design is to be on the craftsman side. It may end up just being spacers.

Zoranaron, I am afraid, the pull-out spice rack would not install properly because you can not drill the pull-out supports into the side of the microwave. I concur with Dytecture, go for a simple filler.

Oh, I never thought of that! Thank you. Simple fillers will have to do. Thank you.

Box fillers will work nice