Has anyone used 'inward tension' using line/rope/long bolts to stiffen a cabinet?

[Spalls]

A simple way to do this is to cut an internal brace piece that spans between opposite panels just slightly shorter than it needs to be (about 1/16 of an inch or so). Position a clamp on the opposing panels and put high quality adhesive on the ends of your brace piece and clamp the sides down to snug the brace. Works for me. It does not take much to put a panel in tension. If you are painting the enclosures rather than using nice wood you can also countersink a screw through each panel into the end of the internal brace to help hold things together if you like. Fill the screw head hole with filler, sand, and you're ready to finish.