I recently added fenders to my commuter/hybrid bike to keep some of the mud off of my butt and bike. The fenders came with exactly one clip, and it did a DECENT job of holding the fender in place, but towards the back of the rear tire, the fender was flopping around a bit more than I liked, so I decided to design/print up some additional clips.
The first design was a straight replacement for the factory clip. In the off chance I busted the original. The second design stripped out the tall vertical mounting point, and just has the two side loops. That's ultimately what I wanted, since the plan was to zip-tie the clip centered over the rear wheel.
Oh, this is also dependent upon you having some sort of bike rack installed, with a vertical-ish post that you can tie up to...
The models are for a 45mm fender, but you could easily scale it to match whatever you're working with.
The design was made in Autodesk 123D, by carefully measuring every aspect of the factory clip, and then throwing all of those measurements out, and eyeballing a 3d model. It fits so perfectly, that there's absolutely no way it's coming off unless I break it off. That's how awesome I am with my eyeballs. For real though, I measured the inner dimension of the factory clip with a digital caliper, and matched that in my model. Everything else was a guess.
I recommend printing with 100% infill. You also may want to print in ABS for a hint of flexibility. However, I printed mine in PLA and it's rock solid.