I had the bike seat of my Dahon folding bike fall off while riding. It turned out that the thread of a custom made nut out of aluminium / aluminum had failed causing the whole clamp to come apart. 

This model replaces this custom nut. All you need is a M8 nut and a M8 washer. 

I printed a test version in TPA (white) and the real version in hard TPU (black). I used 50% infill and supports. 

For mounting, it helps to glue the nut into place. The M8 screw (M8x50, in case you've lost it), needs to be screwed in fairly tight. I added a picture to show that the supplied screw isn't too short (the screw's threads extending over the nut are visible). 

The design is available under the same name on Onshape, in case you want to remix it.