When I first got my tractor, I used a u-bolt with steel backer plate. Those didn't seem to scratch up my paint. I used a wooden carpentry shim between the metal surfaces because I was worried about scratches, too.
I recently purchased the aluminum ROPS Clamps from Ken's BOH. Those are my mainstay, right now.
Long ago, I 3D printed some clamps just to see if I could.
Interestingly enough, my prints were somewhat similar in design to the aluminum version that I purchased from Ken-that being a split rectangular shape and through bolts to draw them in.
Biggest drawback was that they had to be a good bit larger than I liked in order to be strong enough for the job. My plastic version have always been modified to better suit my janky light mount hardware and I added recesses for captive bolts. I've since gotten rid of those lights and with them the need to use the plastic clamps.
I plan to revisit the idea of printing them but haven't the needs now that I have the aluminum clamps.
Note-If you decide to try and go the u-bolt route, if you only happen to come across 4 inch wide bolts, they work fine when clamped across the ROPS diagonally.