Steel may work better if you get the deflection problem
With, the deflection/flex of the jig. it reminded me of a press I made.
I made the press for doing veneers , 6 rows of 6 screws could evenly press a 1.2 x 1.2 area. It worked quite well and pressed heaps better than the vacuum press that I replaced it with, It did flex when I tightened it though and I had to compensate for it by packing it up the amount it flexed . one reason it flexed was the way I made it out of wood, to tighten the thing up I had to do each screw 6 times with a torque wrench.
36 screws tightened with a torque wrench took half an hour , this is why my veneer press is now a nice hardwood set of shelves in a closet at home
If I do another one I'm using steel and a truss design.