The Fish Finder Mounts Down Rigger Adapter Plate with Pontoon Rail Clamp comes with your choice of of a Square Rail Clamp (fits 7/8" to 1.5" square rails) or a Round Rail Clamp (fits 7/8" and 1" diameter rails.)
 
Fits Scotty, Cannon and Seahorse Troll-Master downriggers.
 
Video(s):
 
https://youtu.be/5AnoxNrjWDc