For about $30, you can fortify your bar clamp arsenal with these mighty tools. These clamps apply pressure horizontally as well as vertically, so the joint edges are wedged tightly together and stay level.
To make the clamps, drill 3/8-in. holes (spaced 1 to 1-1/2 in.) along 5-ft. hard maple 2x2s. Then cut two 3/4-in. x 8-in., 20-degree tapered wedges for each clamp. At the hardware store, buy six 5-1/2 in. x 3/8-in. carriage bolts, washers and Wing-Nuts.
To set up the clamps for gluing a panel, slide the boards you’re gluing into the clamps without glue. Then push the bolts into the holes, allowing space for the wedges to be hammered in between the bolts and boards. Slide out the boards, apply glue to the edges, and put them back in the clamps. Tap in the wedges while checking along the joint lines to make sure the surface is even.
P.S. Spread paste wax or put wax paper on the inside surfaces of the clamps so squeezed-out glue won’t bond them to the boards you’re gluing.