You need a system to retain your motor in it's mount. Some motor mount kits will include this, however some won't and if you're making your own from some motor tube you will need to ensure the motor doesn't escape (either way).
A thrust ring at the front of your motor tube, and a retaining system at the rear should work great.
You can choose engine hooks that hold the motor in place, or motor retainers that screw into place.
You can even make a 29mm mount and use adapters to have the option of using both 24 and 29mm motors too, although this will add some weight.