My understanding is most customs are built with a "shell" method.
They make a cast of the impression with a translucent silicone, pour in an acrylic that cures with exposure to UV light, cover the top of the mold and expose to UV for a period of time. Since it cures with exposure to UV and the UV is coming from the outside, it hardens from the *outside*. So this initial exposure hardens only the outside -- a shell. They pour out the remaining liquid (remember they covered the top, so it didn't harden), then expose to UV again to fully cure.
So now they have an empty shell. They drill the hole(s) for the ear canal.
I get a bit fuzzy at this point, but it seems they usually place the drivers and "glue" them in place. Then they attach the faceplate and you have the drivers, basically "floating" in a chamber.
Alternately, I've seen a few which appear to use a tube to connect the driver to the ear canal hole(s).
Personally, I think, like speakers, the size and shape of the air chamber matters, so I suspect that putting a universal into a custom shell might change the performance, and likely not in a positive way. I suspect different designs will probably be affected to different degrees.
That said, with regard to the Ety drivers, while the plastic housing does create an "air chamber" of sorts, it seems to be mostly a "tube". While changing that to a big empty space might have an impact on the sound, I suspect if you keep the general size and shape similar that the sound won't be hugely impacted. In fact, I suspect the more drivers you have the more sensitive they are to changes in their airspace.
It seems that if you put a tube over the "barrel" of the Ety and connect that to the ear canal hole, you could fill all around it in pretty much any way you want. I've thought about using this approach to "cast" the drivers in place. Basically make a silicone cast of your impression, make a hole in the ear canal, and insert the tube (with driver attached) through the hole, and pour in your casting material. When it is hard, you'd just trim the excess tube away and your driver would be cast into the material.
Other thought -- I don't think the Ety "filter" is the same kind of filter they talk about on other threads. I think there are filters to filter out sounds, and their are filters to simply keep gunk out and I think this is the latter type. I suspect the former is more useful when you have multiple drivers, and you want to mute certain frequencies from certain drivers to get an overall balanced effect. With a single driver, it is less likely this would be needed.
In general, it seems the simplicity and design of the Ety makes it very DIY mod friendly.
EDIT: Another thought. After you nip down the plastic, the remaining plastic + driver is tiny. I don't see a lot of benefit to removing the rest of the plastic and using the raw driver in most applications. Indeed, the "tube" from the plastic provides a nice interface to the ear canal hole.
Edited by ccfoodog - 5/27/10 at 12:38am