A minor point to add to this thread:
Yes, put the retention policy in the contract and not in fine print.
Yes, use an attorney. BUT it seems to me, a non-lawyer who has had to use lawyers more often than I liked, it's a contract issue, not an intellectual property issue. That could influence your choice of lawyers. I speculate that there are many more of them well versed in contract law than in intellectual property law. Just a hunch.
The remarks about copyright seem correct to me. But if you are not releasing digital files then retention is a contract issue; your intellectual property is secure enough by virtue of your sole possession.
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