Motor mount (MMT) is just an inner tube connected to the airframe via centering rings. Nothing particularly unique about MMTs to populate a library.
Define the diameter and the length of a tube, pick the material, and you are done.
You can copy and past assemblies...so if you reuse similar setups. Put the CRs, mass object for retainer, mass object for recovery hardware, etc. all as parts "under" the Inner-tube. Then past the assembly into new rocket, and edit positions as needed.
In agreement with K'Tesh and with those suggesting copying parts from already built designs. For my part I created a folder on my desktop/laptop that I labeled "OR transfers". I populated it with my most commonly used bits & pieces (motor mounts, rail buttons, Chute Release, nomex, tracker, etc). Then all I need to do when in design mode it to open one of the files, copy and paste the part I need.
If you mean motor retainers (people have given good responses for actual mounts already) just add a mass element that weighs the same as your retainer. That said, I've never seen one that weighed enough to matter for either altitude or stability.