Antec 900 Computer Case ?

Most motherboards ship with the back io shield (if not all), just push the panel from the back when there is no board in there and it should pop out. if your motherboard didn't come with an IO panel you could make your own using a jigsaw, drill and file and a tad of araldite (glue it to the back) or run it without it, it's only used for grounding purposes.