I’ve built out all of the closets in every home we’ve owned. I’ve done them all with Melamine boards from Home Depot or Lowe’s. Easy to install (with a little knowledge) and limited finish work required.

People on here are mentioning sagging - if it’s the right thickness for the span and supported correctly, it lasts great.

For the width, I’d get 3/4” thick. If possible, support on the sides and back. The span will be fine if you have it supported along the entire back.