You want the notarized document to say the agent is returning both keys and can:
* have access to a specific box,
* remove all of the contents, and
* surrender the lease.
It will need to include a "hold harmless" clause in favor of the bank for agreeing to this request. Your attorney will need to review it.
Treat it like any other access; you will not under any circumstances assist the agent in removing or inventorying the contents.
I originally assumed the customer had moved and forgotten to take care of the box, something that might justify such a manuever. However, you only said that he "lives many miles away." Acceptance of a mechanism like this assumes that returning to pick up the box is more than just an inconvenience and that there is some possibility he would just abandon the box, leaving you with a long term problem. The circumstances need to justify the tactic.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.