I would think that would make sense. I would consider putting in a roll out drawer to give you better accessibility. This is what I did above my washer as I am quite short. The cabinet is 18 in deep like you are considering..

I have a top-load washer. I put a shallow, 7"-deep shelf above the washer where I store detergent and bleach. I put 17" deep shelves above it. My washer and dryer are in a closet so I used wire shelves, not cabinets.

IKEA sells a 30w x 24d x20h wall cabinet (typically placed over a refrigerator). Also 36w.

@Kim G