I've got one that uses 2 large square sheets of 3/4" acrylic, 2 rubber rings, and one section of large PVC tube (12"s internal diameter or so) with a small hole for a regulator and air valve on the side. It's stacked like a sandwich, acrylic sheet, rubber ring, PVC tube, rubber ring, acrylic sheet. To get internal vacuum to start, you usually have to put pressure on the top square. The top acrylic square allows you to view the bubbling as it occurs.