A simple woodwork cramp using 12mm threaded rod to provide the tension. A little experimentation revealed that the 12mm threaded rod was strong enough to prevent distortion when clamping down with considerable force.
The holes for the threaded rod are deliberately tight and may need a little easing out with a round file or suitably sized drill bit. The OpenSCAD is parametrised but the names may not be obvious and some experimenting will be in order!
Use the 12mm rod you have left over from the wine press:
And cut it using angle grinder stand: