Yes, I dare say we all have. It's a very common theme no doubt discussed a dozen or more times here.
They are all tight at first, but loosen with use. My advice is use the stem that fits for a few dozen sessions and try again.
The whole reason folks mess with adding a ring is precisely because the seals loosen up and the stems get looser and looser as the unit breaks in.
If you read back (search on 'tight stem' might help?) you find lots of advice from guys on how they got theirs to fit.
Just enjoy it, it will sort itself out.
The Solo is not intended for concentrates by design, which is why the maker doesn't provide instructions or support this.
In general, it depends very much on the exact concentrate used, and that can vary widely. The thing to be careful of is to keep the melted concentrate from making it to the SS heater body (where it will burn on). But, really, you're on your own here.
There's no way of knowing for sure, which is why they don't publish a guideline to go by. Arizer never intended an extra ring or anything else there. The size of the gap depends on many things that otherwise don't matter if there's no ring there. The exact length of the outer tube for instance (seems pretty constant). And the thickness of the cap (takes up the gap since the outside is at the same point), which is a known variable the maker doesn't need to take extra effort to control in his original design. The over all oven height (again, not a critical deal in the original design) also takes up (or adds) to the gap the ring will sit in.
We decided there was a need for the ring (which is why it's not included with the unit), the responsibility for fitting and use is therefore ours. Way back when, when I experimented with various schems (including springs, flat gaskets, different size and hardness rings and so on) I came to the 113 ring and material that seemed to work in almost all cases. In my own personal 4/20 celebration I sent out little baggies of the proper 113s to nearly 50 Forum members last year about this time.
After that, the thicker caps showed up with fit issues. I got one and posted this photo showing the problem there:
My solution was to stick the 113 ring to a piece of tape and sand it thinner (sand two flats, one on each side, by lapping it on a sheet of 240 grit or so paper on a flat surface). Some guys just didn't tighten the cap down as much, others went to look for thinner rings.
Again, like using concentrates, this is not a supported thing, you're on your own really.
Good luck. I'd go with thicker personally, you can't sand them down if they're too thin and make them thicker...... You could also, of course, simply use the vape as designed, it does an excellent job?
OF