In a 'pinch', you could use shimming.
Aim for a single shim thickness that fills the void but still allows assembly/disassembly to be manageable. i.e. it can be difficult to fabricate a thin shim in this shape/dimension, that does the right job.
Clamps with a 180 deg split may be easier at accommodating shims than those with a single pinch joint.
Whichever way, ensure the shim perimeter is smooth and dag free. Also align the shim ends with the clamp joint ends and that the clamped assembly leaves an end gap in the shimming (and obviously the clamp), to allow full clamp action and strength, when tightened.