I have never cleaned a Cryptochiton, or any chiton close to that in size,
but smaller chitons are very easy to clean if you don't want to save the
girdle.  I just dry them or alcohol-preserve them in a flattened position.
Then I remove the foot and viscera with a small sharp knife.  Then lay them
on their backs on the bottom of a flat-bottomed plastic container of bleach.
The bleach quickly dissolves all the tissue, including girdle, ligaments,
etc., and you end up with 8 cleaned shell plates lying on the bottom of the
container, in correct order.  I assume the same method would work for larger
chitons, though it would take a lot more bleach.  You might want to remove
as much tissue as possible with a sharp knife, prior to the bleach solution.
This method is not as good for rolled chitons, as the plates end up in a
pile, rather than laid out sequentially.

Paul M.