I agree with your process. Requiring the certificate for redemption and holding it in safekeeping goes back to "old school" when CD's were negotiable. As long as you have controls/procedures in place to verify the identify of anyone attempting to redeem, and, for ensuring the hold on collateral, IMO, it's adequate.
If you are afraid that someone, other than the owner, could come in and cash the CD if they found it somewhere, I beleive you have other issues to address! I also found myself looking at nothing but CD's (or pieces of paper) when I would do a safekeeping audit. I quickly recommended removing them and if they just had to take it from the customer for the mental security, put it in the collateral file.
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An error is not a mistake until you refuse to correct it