Here is a mini fender I designed for my onewheel, it is the first iteration an I plan to improve it with time.
20% but 10% should be enough
printed it in PLA for my fist tests, if you are satisfied, ABS would be better in case of water projection ;)
I created only one side of the fender, you will need to make a mirror operation to get the other side with your favorite slicer.
I glued the parts together using bi components epoxy, it seems to work :)