You can use bar clamps, parallel clamps, or a vertical clamping system like Hawk’s to glue up a flat panel from several small pieces, but you have to watch out for the panel warping or cupping. The Plano glue press ensures flat panels by exerting force on both the edges and the top and bottom of the panel.
Advanced Machinery claims that by using their system you don’t need other alignment methods like dowels or biscuits, and the panels come out flat so there’s no additional planing or sanding required to get a flat surface. The clamps mount vertically on a 39″ rail to save space, which also supposedly reduces drying time. You can make panels up to 5″ thick and can expand the system to make almost any length of panel, but they recommend one clamp at least every 12″.
A set of 43″ clamps with a rail runs about $350, while a set of 49″ clamps and a rail cost about $450. You can buy additional 43″ clamps for $120, 49″ clamps for $150, or 55″ clamps for $240. Other configurations are also available from the Advanced Machinery website.