There are two types of fenders: Plastic and Steel.
Our suggestions are:
For plastic, the spool will push down the fender to the tire for a snug fit.
For steel, you could take foam pads, and cut it to wrap around the spool so that the fender will not get scratched.
We will be releasing an addition solution, and are determining the best material for the job.
As long as the tire spools hug the tires ABOVE the horizontal mid line of the wheel, it is secure. In other words, the contact should be above a line drawn horizontally from the axle (above 90 degrees).
NOTE: Sorry, but we are not responsible for any damage to fenders due to these suggestions and your usage of them.