OK, here goes. Please note that this is from a position of moderate ignorance on the subject of what is effectivly medical moulding.
I have a co-worker that has had custom ear-plugs made, much the same as what you describe, but solid.
This was done by putting a small plug in her ear (to protect the ear-drum) then the plastic was injected into her ear, where it set (by what sounds like a chemical reaction).
I have recently begun experimenting with a material called 'polymorph' or instant mould. It is a thermoplastic that becomes soft at 62 degrees celcius, but remains plastic while it cools. The important part is that once the plastic is cooled, it can be machined, drilled etc.
My approach (remember I am just floating ideas here) would be to use a sufficiently cool lump of polymorph to firmly plug the ear canal (being careful not to push it too far in) and adjacent ear structures.
Once the plastic is hard, it should be (carefully) removed, then can be drilled etc to allow for the speaker components. Once the part is finished, then you could either use the finished part (as my co-worker effectivly does) or make a silicone mould so you could replicate the plug as needed.
In this case, you would not need to vacuum the silicone (although I would, but I have the equiptment already
), just use pressure to ensure a good mould.
Then you can cast the plug as often as you need. In whatever material you want.
Please remember, I am not an expert, and you should be VERY careful doing anything with your ears.
I'll be interested in hearing what others think of my idea.