You can do this by doing as little as making a single file named LICENSE in the top level directory, with a single line:
This work is licensed under CC BY-SA 4.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/4.0
That's it.It is the hardware/artwork spritual equivalent of GPL2. It means the user must not remove your name, must make source/plans available, and commercial activity is ok.
Just a suggestion if you don't know where to begin or what to do, hadn't really thought about it or read up on all the infinite options etc.
Awesome. Thank you for releasing your work as open hardware.
Without any declared license the default is full restrictive rights to the author/artist/creator.
That is how literally true it is to say "can't do anything with this" they can't even view it.
Let alone actually download a copy, let alone build it, let alone modify it, let alone redistribute their modified version...
I don't know what this "transactional" accusation is supposed to even mean, but the need for a license is completely reasonable and not asking for anything.
My own "jesus" question: Jesus do you do any sort of work in either software or hardware without knowing this?