If you decide to get a Rotopax gaspack to mount on your 550, get the 2 basic mounts, all you need. I have the optional front box and it is reinforced pretty good. Postion the gaspack on the front box, use a pencil to mark the mounting positions, drill the four holes with one size smaller than the 5/16 (makes a tighter fit). Use the concave plate that comes with the mounts for the backing plate inside the box and 30 minutes later it's mounted. I can't take the credit for coming up with the idea but I ain't giving it to my buddy either...lol.