I’ve only seen these billboards on eBay (I especially like Crappy Barrel), so don’t totally understand what they are like (they appear to be similar to an empty box of Good & Plenty), but I have a couple of ideas to throw out.
If you don’t want to cut down the box, you could put a metal thumbtack into the cubicle wall, and then put one of those small super strong magnets inside the box to keep it in place – the back panel of the box being held in place between the magnet and the metal thumbtack.
If you cut them down to be flat, they could be displayed in an oversized top loader. One method I use to display photos and Wackys is to mount a top loader onto a piece of foam board. I stick a piece of foam board to a top loader (with either hot glue or a loop of tape) and then attach two thumbtacks to the board, with tape, points outward). This allows you to attach the top loader to a wall, bulletin board, or anything else a tack would stick in. Instead of two thumbtacks you can use adhesive magnets. The foam board raises the top loader off the wall, fridge, or file cabinet a ¼ inch and looks sharp. If there is a discrepancy in the size of the top loader and the item (as in my example below) you could cut a piece of colored paper to fit the top loader (5”x 7” is a bit too big for the postcard), and slide the paper behind what you are displaying… a poor-man’s matting, so to speak.
[expando]http://i1184.photobucket.com/albums/z332/rbishop224/Foam1_zpsb6560f5b.jpg[/expando]
[expando]http://i1184.photobucket.com/albums/z332/rbishop224/Foam2_zpsff7614d5.jpg[/expando]
If cut down flat, there are various sizes of First Day of Issue Cover / postcard binder protector sheets (I’m a stamp collector), that might work to display them. For example, this sheet has two horizontal pockets that are 9 ¾ " x 5 ½ ", but I think they may be refills for a special oversized stamp binder and might not fit correctly in a regular binder. There appear to be a few various pocket and sheet dimensions of this type, for displaying blocks of stamps and so on. There might be a version that is 8.5" x 11"
http://www.ebay.com/itm/SuperSafe-First-Day-Cover-10-Horizontal-2-Pocket-REFILL-Pages-Refill-Pages-/310374017431