I picked up a floor model "as is" floor model type sand blasting cabinet from Princess Auto, CDN version of HF. It was only partially assembled and it had a couple of parts missing but I still got a VERY good deal on it. However, I do have a question; there is a plug for the vent on the back wall behind the baffle. So is the hole ,about 4" in diameter supposed to be open or closed with the plug? Thanks.