Custom tips must fit right to isolate properly. (Most do not)
1. This is only my opinion.
2. I am biased, like everyone else.
3. I have not heard everything in the world.
As the younger brother of "Dr. Abonso" Mead Killion the founder of Etymotic Research (of ER4 fame), I've been blessed to enjoy ER4, ER6i, etc. state-of-the-art audio from the beginning. Something I never was able to accomplish, however, before he sold the company (and much of the website technical and historical documentation began to fade) was the offering, by Etymotic, of state-of-the-art ear sealing for those of us who can't endure the discomfort of the stock Ety offerings.
What I consider to be this state-of-the-art ear sealing is the marriage of 3M 1100 foam seals (soft, fine pore, high damping) and Etymotic products including ER20XS hi-fi attenuators. EAR made pioneeringly high damping, thus high isolation, foam plugs and these were adopted early-on for Etymotic Research production, but the coarseness of the EAR foam made them just as discomfiting, after a very few minutes in my ears, as the 3-cup seals' line contact. I simply couldn't bear wearing either for more than a few minutes before my ear canals were screamingly itchy.
I learned late in the game that a 3M salesman had somehow deceived or double-dealt Brer Mead at some point along the way, which explained his resistance to the use of 3M seals.
My happy (high damping, so good for lawn mowing etc. isolation, and smooth, for hours of insertion comfort) personal solution was adoption of the 3M 1100 seal, which I recognized as having been adopted by the airline industry for passenger handouts; thus essentially representing the de facto gold standard of the industry. These (3M 1100/Ety mounting tube) seal "assemblies," which have limited life before succumbing to the stress concentration at their interface with the mounting tube, can be made in batches to enable ready swap-out as needed. I use a batch of tapered wooden mandrels, created from 1/4" shish kabob skewers chopped into ~2" lengths before being hand drill spun against the RH face of a bench grinder wheel to produce a ~1.5" long taper to ~2mm, as "freezer fixtures" to enable sufficient assembly time between 3mm pierced seals and mounting tubes made from 1/8" OD fuel line from the hardware store. The 3M 1100 seals are shortened by chopping ~10mm off their blunt end, then punched with a leather punch after axial compression into a pancake, the trickiest part. I pre-mark the dome of the seal at its tip with a Sharpie dot to provide a visual centering device for the punching operation. After the seals have grown back from their pancake state they can be "screwed" onto the mandrels butt face first using parallel fingers on the OD as driver means. Pop them into the freezer for ~20 minutes, then apply superglue to the OD of ~1/2" long tube segments one at a time (grabbing a "frozen" seal/mandrel assembly from the freezer one at a time) for quick twisting dismount from mandrel then quick installation to flush (to 1mm recessed) at rounded end over the mounting tube. Finally, after the superglue is cured, mount the assembly on a hand drill-driven tapered 1/8" shish kebab skewer for spinning against your bench grinder wheel to taper, and thus minimize ID stress concentration, the foam from OD of mounting tube to OD of seal. See pic of vintage ER4 phones (which have made many trips abroad exceedingly pleasant), for visual example.
https://drive.google.com/file/d/1ey8bdVwmZ7Bj4iMS1qDSwUn0mMbHhaAS/view?usp=sharing
