[PugslyCat]

I did a search for "rear fender torx" and ran into the thread Fender Bolt Tool. In there we seem to have determined several things:
1) The 2015 bikes used an Allen wrench, possibly 6mm size.
2) A couple of us were fairly certain it's a T-45 Torx from 2016 on.
3) One member on another thread insisted it's a T-50 Torx.

My suggestion is to get an inexpensive set of Torx sockets with both just to be safe.

 

[PugslyCat]

Yea, in that thread I also say that it's a T-50 based on another thread. I didn't link to @hillyeja making a post to that fact in luggage Rack help and probably should have. Since @skeive and I both initially thought it to be a T-45 I probably should have stuck with my original statement.