As the Touch Plus controllers don't have a ring, they would rattle around in the Syntech Medium case I bought for my Quest 3.
This insert fits in the center part of the Syntech case and hold the controllers similar to how the official Meta case does.
No more rattling during transport.
For the controller's negative shape, I used Paul_delta's 3D model : https://www.printables.com/model/625580-meta-quest-3-right-controller
EDIT V2 :
The author marked this model as their own original creation.