Hanging microwave without above cabinet
You're essentially creating an inverted shelf over your microwave. Just adhere to all the principles of mounting a shelf:

Adequately strong brackets
Anchors of appropriate size and depth
Shelf material that's up to the task

I would not use brackets of that type. They don't seem robust enough to me. Heavy gauge steel with a diagonal brace would be your best bet. Two &quot;max brackets&quot; mounted upside down with a 2x6 or 2x8 screwed well to them would do nicely.