I had a similar problem but unfortunately mine was out of warranty. While I bought a new part I tried many adhesives to glue the old piece back together again before I got the new part. The only adhesive that seemed to work, and it worked very well, was Crazy Glue (or generic equivalent). I actually debated not changing the part when I got the new one but eventually did.
The shaft that holds the fins has a flat spot where the set screw is supposed to seat. It is difficult to know if the flat is in the right spot. I used a hacksaw to create a screwdriver slot in one end of the shaft that is parallel to the flat. Now I know if the shaft has spun if and when the set screw comes loose. I believe Hobie recommends putting blue Loctite on the set screw when inserting it. Do not make it too tight, you will split the housing!
I'm glad to hear Hobie will cover yours under warranty but I was surprised at how inexpensive the part was, like $25 or less, which is amazing when one considers a Turbo fin is about $20 without the mast.