I made a blank map that can be used to create a setting collaboratively. I hereby publish it under the CC0 license, it's the closest I can get to the public domain without breaking any law (or dying). Feel free to share, reuse and remix it as you see fit!
Here are two examples of completed settings (in French):
For best results, I would advise to ask your co-authors to put references in their contribution to elements that have already been established by others. It really ties the setting together.