This cap was designed to be compatible with Memoire's Perses Hopper design (found here:
https://www.thingiverse.com/thing:3991277).
This design utilizes an M3 bolt and nut to clamp the cap ring onto the hopper (as well as serving as the axle for the cap pivot).
I have uploaded both the STEP and STL files for this design for easy modification.
I printed Memiore's hopper out of PETG, and this cap was designed around the "as-printed" dimensions of that design. Depending on your specific printer and material selection, you may need to slightly modify this design.
This cap and ring were printed out of PLA (because that happened to be the material I had in the color I wanted).