Typically, a custom rear fender
will have four mounting positions to help ensure the fender does not break. These would be a low mount, mid mount, and then a mount on either side of the fender near the rear of the bike.
These rear mounts could be in the form of a sissy bar
or fender struts. The number of mounts is not really as important and the location of the mounts. You want to make sure the fender is evenly supported.
It’s also a good practice to make sure when you tighten the fender to the supports, that you tighten the mounts evenly. Doing this will make sure the fender is evenly supported and if there is any stress on the fender it is evenly distributed.
Also, the fender should not be under any tension or it will eventually crack. For instance, if you have to push a fender into position to get a bolt in place, it would be better to turn the mounting hole into a slot to allow the fender to sit without tension when inserting the bolt. This will help ensure durability, and avoid hassles on the side of the road or while riding!