re: Making a form for an arch

You will develop substantial force with the laminations. I would bite the bullet and make a solid half circle (a little more than half actually) of two layers of plywood with spacer blocks to give you a little over three inch thickness, then cut several holes as required to allow access for clamps. The pieced up mold may work but if it doesn’t, you will find out too late. With a solid mold. You know it won’t move.