Could a person make up their own chaplet if they wanted? If so, what rules must they use? If not, why?
Yes I would say that a person can for personal use…(though I would think that one could privately share with say a friend or family member…that sort of private Christian sharing making sure that it is stays in the family or with ones friends and they do not think it is some new chaplet to start promoting…).
After all they are usually simply the counting of some ejaculatory prayers with some beads…(and one can make up as many as one would like!) (and perhaps if one would like…adding some theological truth to meditate on or contemplate)
However promoting such publicly as a chaplet I would think may require approval from ones Bishop.
I know for sure one needs permission to publish books of prayer:
§3. Books of prayers for the public or private use of the faithful are not to be published without the permission of the local ordinary.
I agree with the response here - I would be curious to know what the OP has in mind. I have thought of composing prayers at times…