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.

Status:
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:
1: print
2: attach a plate to the upper end.
(not part of the design because the overhang would be nearly unprintable.)