Thanks for those pics also D, as I get the LED leg sizing part too. I thought before that the square leg's corners were much tighter inside the round needle. It's clear now.
Also a longer needle for cartos isn't really needed, as all the air comes in the bottom of the carto via the 2 notches. Get the needle in the way and it stiffens the draw (like the 18G, and why I like it) or blocks it from airflow completely (as in a 16G, not having been 'fixed').
Still, having some play in needle height is good to help control the flow.
As for the ring, I like it, but it does need to be airtight, IMO. A silicone carto cap tube would be good for Dan's, for the 1/2" HF fob, how about clear vinyl tubing? It would add a filling port, juice window, and possible airflow source. I'm not quite set to drill in the side of my 2 as they are set too low in the devices, There's either no room and/or too close to the threads.
Personally, I'm liking Dan's needle mounting design mixed with my control valve connector, and somehow a 16G needle.