get another board about the same size as the spreader but an inch longer.
Lay it on the existing spreader bar and drill it out about 1/3 along the original with the longer inch on the short side
put a bolt through as a fulcrum.
Put the long end of the original spread on one gunwale, the new board on the other and lever it open like a scissor jack
Maybe you would want to put another hole for pin to keep it held open while you put the middle seat in.