I just use some bits of 25x8mm flat with one clearance hole (for m8 thread, but it could be a slot) and another tapped hole near the end. I then just have a set screw in the tapped hole to set material thickness and you would need a tee - nut in the centre (mine are fixed round the edges as I only use sheet material. Basically the same as you show, but more versatile as it allows greater flexibility for material thickness. these clamps have cost virtually nothing, but are priceless!!! A wingnut locks the adjusting screw in position if necessary. G.