Perfect application for owning a 3d printer:
Saw other cyclists having this hot mudguard on the back of their saddle. Looked it up and it costs about 10$. Since it is just a piece of plastic I asked myself - why not print my own version of it?

There you go! After some iterations I managed to find a design which fits just perfectly into the frame of the bottom side of the saddle. This design totally saves my ass on a rainy day! Since the dimensions of the bike saddles frame are standardized, this mudguard should fit to your bike as well.

Mounting:

Make sure to carefully slide the mudguard into the frame of your saddle. Check out the last picture in the gallery, which shows the bottom side. You will realize that you first need to clip both rods into the provided gaps and then slide the nose of the mudguard into the thin slit of saddle mount. Moving the whole mudguard a little to the front will slightly tilt it upwards, which makes your bike look really hot!

Have Fun saving your ass and share your makes, I would love to see other colors on other bikes!

UPDATE

There are two versions now:
bike_mudguard.stl -> is the original design with a total length of 279mm, which fits perfectly on the printerbed of the prusa i3 mk2 if you position it diagonal
mudguard_short -> is the new and shorter version for all smaller printbeds. With a total length of 250mm it is designed to fit to the print bed of the anet a8.