Note that the current fins are significantly smaller than the previous version; smaller tip chord, greater trailing edge sweep. I don't have an .rkt file, unfortunately.
Mine was built, using the fin design, similar to the Bruiser fin. I just need the basic file so I can manipulate and work sims. If Tim hadn't screwed with the change over from 9 to 10 versions, wiping out all of my files when he crashed 9 perilously, I and others wouldn't be in this fix. Motor files are also gone.
I have been using RockSim for many years. I got my first copy on 2.5 disks, probably have them somewhere in my desk. Still have several versions on CD disk also. What I may be able to do is find it on one of my old laptops. Hopefully some of my older sesign projects will also be there, including my L-3 project, an upscale of the RocketMan Wahoo on 7.5 glassed tube.