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?
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.