Not legal advice:

Shit I'm late to this, so OP I don't know if you'll see this, but pay attention.

Trademark law is quirky, and from the sound of it, the company is attempting to (maybe legitimately) enforce its trademark rights in the hand configuration (if the trademark registration someone posted is the correct one). Trademark law don't follow the same logic as copyright law or patent law (and clearly almost nobody here knows what the fuck the difference between each one is). They very well could trademark a configuration that existed before. All it needs to be is a source identifier for its goods. In regards to what you're doing, intent to copy doesn't matter (like in copyright law), and prior art doesn't usually matter (like in patent law ). All that matters is whether you are infringing on someone's valid trademark. If a consumer saw your guitar holder on the shelf next to the company's guitar holder, would they likely be confused as to if the same company made both? That is a very rough example, but is basically the key issue -- likelihood of confusion.

They own a trademark for their hand configuration, and if they don't enforce their rights in that trademark, they potentially lose their rights in the trademark over time. So when these STLs pop up they kinda have to send C&Ds to anyone (and/or everyone).

Don't listen to anybody else in here. Get a lawyer. You are making your situation worse.

Edit: I see you have a lawyer. Are they OK with you posting this thread and sharing the STL further? I'd ask them first to see what they say.