What are the current rules/regulations/laws regarding storage of NFA items at a residence not of your own? So far I know that one needs a locked case with the trustees only having the key/combo for access. John Pierce, Esq (not sure how reliable he is with NFA information) recommends giving the individual storing the items a copy of the F4/F1 and trust as well.

From ATF FAQ for a similar situation:

"The friend or relative should be supplied with a copy of the registration forms and a letter from the owner authorizing storage of the firearm at that location."

https://www.atf.gov/firearms/qa/if-individual-changing-his-or-her-state-residence-and-individuals-application-transport

I don't believe there are any safe storage requirements in the NFA, though it may fall into the area of good advice.

You do not want to be negligent in allowing prohibited persons access to NFA items, or any firearms for that matter.

Your state laws may differ.

ETA: The above post is correct but not compelling, much like other guidelines from ATF.

I believe the OP was trying to avoid an inadvertent/unlawful transfer of an NFA item.

I would consider the ATF guidance to be one way, but not the only way to demonstrate that no transfer was intended or did it occur.

I speculate that you are correct, in that there is no safe storage requirement for NFA items but there is a sever penalty for an unlawful transfer.  This would include the constructive possession by a friend or family member.

This is not just a matter of avoiding a transfer of a firearm to a "prohibited person" it is a matter of preventing a transfer to anyone not authorized to possess a particular NFA item.

If I misunderstood or was confused by your post then forgive me.