You've probably solved this by now, but for anyone else searching, I found a partial tear down here:
[link|https://fccid.io/2ARXN-FC1/Internal-Photos/2ARXN-FC1-Internal-photos-4194665|https://fccid.io/2ARXN-FC1/Internal-Photos/2ARXN-FC1-Internal-photos-4194665|new_window=true]
I'm going to try to 3d print a new case. I just started researching the case. I'm going to take it apart and get some measurements. Might take a couple tries to get the design right, but I'll follow up when I can. There are a few on Thingverse, but I didn't find what I was really looking for.
Thingverse Fi Collar Results: [link|https://www.thingiverse.com/search?q=fi+collar+&page=1|https://www.thingiverse.com/search?q=fi+collar+&page=1|new_window=true]