Shamir it.

please wrap the whole thing as a trustworthy product

I don't understand why this was downvoted. In case it's not clear: (S)he's saying to split the key into multiple shares that can be used to reconstruct the key if you have a large enough quorum. Then store each share in a different place. As long as you don't lose too many of the shares, you'll be fine. And one baddie is NOT enough to get the key.

Either shuffling those keys stored in N different deposit boxes is overly complicated for a normal person, or it is not overly complicated for a moderately dedicated baddie either

Unless the "baddie" in this case is the government, why would it be easy for anyone to obtain access to multiple secrets stored in multiple boxes/banks?

Multisig is a pretty common setup for crypto and there is software that makes it easier.

Can you show how it can be easy to use in normal life for a regular person and at the same time really difficult for the attacker?:)