My apologies for the very slow reply to your query, I've only just revisited this post. Hopefully I can answer your question by explaining via the attached photo.
Yes, the internal diameters of 1 and 2 need to be enlarged to allow the (pared down) micro HDMI plug to pass through them. However, the hole at the end of 2 needs to be kept small enough to retain the s/s washer 3.
Washer 3 is cut and twisted to allow it to be placed over the cable, then twisted back into shape.
I was able to stretch the rubber seal (4) over the micro HDMI plug, once the plastic sheathing on the plug had been carefully pared away. A little soapy water on the seal probably helped.
I've never had any leaks with it, the cap (2) can be screwed down quite tightly onto seal (4) to ensure it's waterproof.
The only problem I've had was where the cable emerged from the cap (2). It's a flex point and the wiring eventually failed. I addressed this by adding a piece of rigid 90 degree plastic tubing (from a garden irrigation system).
I hope this helps but let me know if you need more info.