a husband or carpenter can use small L brackets ...looks to be a smaller mwave. which means lightweight. If that is to be the case for the future a fixed spot for shelf placement will be fine. when you have a full sized unit and so on there would be different issues.

is this what you mean? i think your appliance is even smaller than this.

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That’s nice. How do you attach the L brackets without the screw going thru the side panel of the cabinet. And yes my microwave is smaller.

show the full cabinet. you get the proper sized screw.

Here is a better picture

a stringer piece of wood can also be applied to sides and your shelf rest on that. We have such a set up inside an old pantry cabinet but good spot in kitchen.... fixed position is just fine because it's perfect height and smaller light weight unit is all that is needed.....about one cu foot mwave for a part time use home. figure out what you want for future appliance size and features...if the lower cabinet will be a permanent spot or not .... etc

Look inside the cabinet for shelf pin holes in the sides.

Are these new cabinets? Have you asked your cabinet supplier?

Thank you both. Unfortunately this cabinet had no holes inside. It is a microwave cabinet but it’s made for a much larger microwave.

I’ll see if my husband can do a stringer piece of wood.

" It is a microwave cabinet but it’s made for a much larger microwave."

It looks like that MW barely fits in that space from side-to-side so doubtful that a larger MW would not fit. Judging from your photo are you looking to just raise the MW about 4-6 inches higher within the same shelf/space? If so, then as John Liu suggested a couple of short 2x4s or 2x6s painted black placed under the left & right sides would raise it up a few inches. Keep in mind that 2x4s are really smaller as are 2x6s by 1/2 inch.

Thank you that’s something to consider