A simple blast gate I made up for my dust collection system The gate slide has sandblasting mask on the suction side so it seals really well when closed.
http://i.imgur.com/tdjR9N1.jpg
http://i.imgur.com/7RBiMPS.jpg
http://i.imgur.com/nP6NvnH.jpg
http://i.imgur.com/rzdJR4B.jpg