I'm glad to see attention drawn here. The only way to get any reasonable sub 500 Hz response out of tiny in-ear transducers is to have a good seal. The standing waves go a long way. Molded in-ear monitors are best, but they're speciality items. It would be cool to see a modern true wireless earbud you can order and ship them your ear molds.
There is actually a subreddit called DIY in ear monitors. https://www.reddit.com/r/DIEMs/ You will be quite impressed and I suggest you take notes and dump great posts into OneNote or something similar to index, so when you make your own, you have a great reference and cheat sheet.
Edit: Oh I forgot to add that I also got tiny microphones and lodged them into the IEM blackplates for ANC or pass-through. It's an ambitious hobby project. I'd be happy to see a proper company do it (or at least incorporate elements of it).