The 8.5x11 acrylic stands can be very nice for displays, especially the ones that lean back at an angle. But they can get pricey if you need many.
One thing - first decide if you NEED to display the certificate (well, usually a copy, marked as a copy). Some certificates are great, clear, have the company logo. Others use small text and aren't easy to read as you pass by. Instead we often create a
flyer with eyecatching clipart, colors, and, details highlighted in large font.
But still -then how to display the flyer? If you can manage what you need in half sheet, printing on cardstock and folding as tent signs is super easy, cheap, and takes minimal space.
I also find essential a bag of acrylic clips about 3" x 2" and serve as a stand to hold placecard, signs, etc. Usually you find them at a trainer or conference supply source. Sometimes Amazon.
PCNametag
carries them. (Works better for 8.5x11 size turned
landscape. Sometimes you need to fold a piece of paper/cardstock to create a "spine" to help the flyer stay upright if portrait orientation.)
These are sold by the dozen and I keep a bag of 2 dozen in my supply kit. They are super handy for events, serving many purposes.