This triple fan mount is completely overkill but it works well. The newest version has full height tabs along the rear edge. Much better fit than the original. There is also a single fan mount. It is rather bulky for only holding one fan but it is designed to be attached and removed quickly to make transportation of your bot more convenient.
You probably want to use a quick disconnect connector to connect the fan(s) too.
This clip is of V1 of the mount but the use is the same.
Your tolerances must be pretty tight. This thing is held in place mostly by friction. There are also two tabs on the back that should hold it in place. You may want to adjust the two extruder mount nuts so that they stick out as far as they can. This will provide more for the mount to grip to.
UPDATED to V2 (Fan Mount6 & Fan Mount7)
Much better fit. Full height tabs along the rear edge that lock onto the main extruder support.
Note: I have not actually printed the single fan version. It is the same design as the triple mount but with the side mounts removed so it should print fine.