I wanted a G-clamp to use to mount a power strip onto my workbench. I needed a different size though, so I redrew johann517's G-Clamp fully printable in Fusion 360. The Fusion 360 links for those who would like to modify:
The clamp body: http://a360.co/2dSL9aJ
The screw and pad: http://a360.co/2duKGcl
If you go into "Change parameters" under "Modify" you should be able to change some things like the length of the screw. I'm a bit of a CAD noob though so there are probably limits to how far you can change just the parameters and have some of my free hand work line up. You can walk back through the history though and change stuff that way.