We use something that is uber cheap, but looks clean/decent. It's just a $8 wire shelf piece that one would use in a closet...it's super light weight, easy for humans to step over, but keeps the doggies at bay. You can get them to be 12 in high or 16 in high...the 12 in work for us fine.
You can cut them to correct length yourself using a bolt cutter, or, Home Depot can cut them for you. You can spray paint them any color to blend in w/ decor.
Like this:
ClosetMaid SuperSlide 72 in. x 12 in. Ventilated Wire Shelf-4717 - The Home Depot