Many portrait photographers who use a white background for 'high key', use tile board instead of plexiglass.
Tile board is less expensive and not quite as reflective as plexiglass. Since tile board is white rather than clear it doesn't show scratches as readily as plexiglass does.
Tile board has fake tile on one side but the back is white. You get tile board at a home supply or hardware store.
Obviously, if you're using a colored background plexiglass is your only option.