I redid this from scratch based on an original Canon lense-cap.
It has more details that the simpler copies from other lense
manufacturers not have.
It is also tapered to stay fixed in the lense.
I DID NOT PRINT THIS YET!
I hope to print this next weekend.
Done in Alibre Design. (there is a 30 day trial that can handle this thing)
Make a lense-cap:
2: attach a plate to the upper end.
(not part of the design because the overhang would be nearly unprintable.)