I use small o-rings. Got them from a plumbing shop, but like
@Titienne said you can get the small rings used to dampen keyboard keys. Cheap from Amazon.
I've been wondering the same thing. It's so simple. It should be mandatory really. Or at least use 'grippy' paint on the nozzle instead of high-gloss paint that makes them tip-resistant!
#firstworldproblemsiknow
Edit: if you use the regular Sedna EarFit or Light tips you don't need a spacer because it seems Sedna understands the struggle of those with long nozzle IEMs and make their tip stems long enough to allow for a decent gap between tip and nozzle. Which makes me wonder why they didn't make Xelastec with a standard stem length...