I am in a very similar situation where I am working on a non-commercial game and looking for unpaid artists. HOWEVER, unlike OP, I don't mind the artist retaining rights and reusing their graphics/music in their portfolio or even other projects, provided they do not end up giving an exclusive license to someone else that would retroactively affect the game.
Basically I just want to avoid either someone pulling out of the team near completion and taking the art with them, or us having to pull down the completed game in the future because they decided to give exclusive arts to the same art to someone else.
In this case, do you think a formal contract is warranted, or would it still be going overboard given the noncommercial and unpaid nature of the project? I was thinking of something like complete, perpetual and irrevocable, but non-exclusive license, with the artists retaining the original rights. Should I go straight to an attorney or are there any good online templates I could use (still looking through the links posted in the thread so apologies if I missed what has been already posted).