From FAQ – Purism :
Will this be an “open hardware” design?
Our intention is to have everything freed down to the schematic level, but have not cleared all design, patents, legal, and contractual details. We will continue to advance toward this goal as it aligns with our long-term beliefs.
It does not answer your question exactly but I guess this may mean they will free the case as well.
This may also be interesting: Phone Case for Librem 5! Who could make a great case? - #48 by ecs