I have open back buds, but they not have such a big vent hole. In the case like this I will probably combine a couple of thingsYes I've seen the diy bud with your dream shells and the 400ohm To be honest, the finish doesn't look so good, the edges are quite rough on around the back vent for example. By the way, how do you tune a shell like that reliably? What sort of material do you put on the vent? I never had an open back shell before, so my main concern is how do you get reliable channel balance, how do you put something on those vents that covers the whole thing tightly and wouldnt get loose after a few days or weeks.
(we're talking about this shell https://www.aliexpress.com/item/1005001773416768.html and this earbud https://www.aliexpress.com/item/1005001836926242.html )
First I will add another "dust" steel mesh from inside to block air a bit more https://www.aliexpress.com/item/32800734441.html
Instead of steel mesh you can use small pieces from something like this (only hook) part without adhesive https://www.aliexpress.com/item/4001052968496.html
Another layer will be cotton filter (round part) https://www.aliexpress.com/item/2046606985.html , or this inner part https://www.aliexpress.com/item/4000250823937.html
More of this but for headphones https://www.aliexpress.com/item/2035370421.html
At the end if this is not enough you can always stuff the cavity with some absorbent material (sponge, mop ...)