lee drennen: The new wheel chocks held the trailers better but the front end of the trailers would still slide from side to side.
I then made a chock for the front also and it was a lot better. I then discovered that with the front tied down the original chocks worked OK.
The front chock attaches with a 6-32 x 3/8 in screw and nut through an existing hole in the flat car.
I have attached an (.stl) file that can be 3D printed if anyone else would like to try it out.