They made them sprecifically for the 500's back in the day.  There was actually a NOS one on eBay a little while back.  I almost bought it, but couldn't decide if I'd like it or not.  Looks pretty good in that pic though.

Edit, here it is....says LT 500 Quadracer right on the sticker:

http://www.ebay.com/itm/Suzuki-plastic-number-plate-NOS-LT-500-Quadracer-1987-88-/261081269274

Someone buy it before I do lol