i'm the lucky one, i've had 4 pairs of custom IEMs made and these are my experiences:
Westone ES3X: had impressions made locally, perfect fit
Westone ES5: they used the same impressions again, perfect fit
JH Audio 13: had impressions done by JH at CanJam. fit was very good but not perfect in right ear compared to Westones. I mentioned this to JH once and they were going to fix it. shipped my JH13s back to wrong address, lost...
JH Audio 16: got these as replacements for the lost 13's. I sent them my Westone ES5 to use as impressions. Perfect fit.
so there's an idea. get impressions done, use them to get custom ear plugs or something like that. if they work for you, ask JH if they can use them as your impressions.