The part on the outside of the servo is custom fit to this model servo, but it's also an optional part and should fit a lot of other servos that have the same "face" shape.
Heat sinks are 15 x 15 x 6.5mm, and are optional but not using them would greatly reduce the amount of sound deadening you can use. Without heatsinks, fully enclosing the servos with TPU (or another substitute material like foam) would cause them to get too hot. Fan is going to be required for this, or at least, as required as I can make it. I'm not your Dad.
I'm also planning to include a spot to run a KSD9700 Normally Closed temperature switch. Having it in line with the servo power gives a hardware kill switch with a customizable temperature (by choosing your switch's temperature activation point) that doesn't rely on a controller being on. I can't make you use one, but they cost a few dollars and can keep you from ruining your toy. And your toy, too.
I'll be making an aliexpress list for every nonstandard part I use. I'm just going to assume everyone has Some of the Usual M3 screws, but beyond that I don't plan on using much non standard stuff. So far:
5150 5v fan (1 or 2)
KSD9700 Normally Closed switch (60c for now, idk)
15x15x6.5 heatsinks (4) but I plan to make room for plenty of common sizes, if you have any of these laying around in a drawer it'll probably work.
I'm just going to dump some photos here of the concepts I'm working on for this part. If you can print TPU, start drying out your roll because I'm going to make custom fit gaskets and vibration absorbers for this, which should work even better than the mouse pad I sloppily cut up for testing.

This is basically just single extrusions alternating and then doing the same in the other axis on the next layer. It produces tiny gaps which are, according to a research paper I saw, extremely effective at attenuating high frequency noise. I have no idea if it's any good, but once I've got a final shape, I can theoretically just make a big block of this stuff in fusion and then cut my model out of it. If my computer doesn't melt, that should be a perfect fit.

A hacked together temporary gasket made out of an old mouse pad. TPU gaskets are intended but if you can't print TPU (you probably need a direct drive extruder) this kind of material will work as well.

I'm thinking that this part can have two screws on the outside and two screws at the servo and it should be a nice combination for keeping them completely stable. Definitely not going to have plastic between the horn and the servo, that would be silly, but it's satisfying that the tolerances worked out this well.