Just to note what I found out doing my impressions last week w/ a JH approved audiologist, it seems there are 3 things that may be different from a typical hearing aid impression:
1) They do a musician fit, which is past the second bend of the ear canal. It's quite deep and at least for me was pretty uncomfortable (a nerve was was hit that felt like a needle was poking my eardrum, lol)
2) A bite block is required at the tallest part. You have to hold this position with minimal stress which might be hard for some.
3) The impression extends to the outer portion of your ear canal.
JH supplies a document indicating this to be shared with your audiologist. Personally, I didn't see how the procedure required much skill to do, just have to be mindful of this stuff. It also seems from what I read in another thread on taking impressions that many people screw things up for themselves by biting down too hard on the bite block which causes some occlusion in the ear canal.