Pardon our mess
Ideally, having you come into the shop so your piece can be fitted would be best. However, chances are that you won't be able to make such trips because we know you're a busy person. Solution; casting is the best way to ensure a perfect fit. The process is quite simple .
Casting can be done on any part of the body and with a few different materials. The process allow us to "copy" the part of the body we are building a piece for, and have that copy of you in the shop so we always have you there. Added benefit, the cast doesn't need to wait for the piece to cool during the check fits.
I prefer castings over measurements only because numbers don't give me a true three dimensional reference.