For flat panels, strut’s your stuff
I used to struggle gluing up flat panels. Now I use common strut channel (often referred to as Unistrut, a major brand), to hold boards in the panel perfectly flat as I clamp them.
Home centers sell the U-shaped steel channels in 10' lengths. Cut those into pieces a few inches longer than your widest glue-ups, arrange the channels and hardware as shown, then tighten the wingnuts just enough to flatten the glue-up. Add clamps and bring the glued edges together until you see squeeze-out. Feel the edges of the boards, and if they shift slightly out of flush alignment, tap them with a mallet. Tighten the wingnuts a bit more if necessary. With woods high in tannins, such as oak and walnut, place waxed paper between the channels and the wood to prevent staining.
—Dennis Sheehan, Antioch, Ill.