This 0-2"/50mm rubber-band powered spring clamp is similar to this clamp, but it prints flat for much greater strength. The two sides print as separate pieces that are held together by your rubber bands. This model scales nicely: shown are prints at 100%, 75% and 50%. Just snap the clamping pads into place, hold the 2 sides together, and add as many rubber bands as you like to get the desired clamping force.

Printing
I suggest printing in PLA with 0.2mm layers, 3 perimeter lines and 15% infill should make them sufficiently stiff. If you decide to scale down the right an left sides, yo may need to scale the pads down to just a hair smaller to get a good snap fit. e.g.: if you scale the sides to 50%, you may need to print the pads at 49.5% (or so).