Had to do the same thing with a 99 m900 and a Sargent seat, but the m620 did not have same problem.I had the same problem with a Corbin seat for my 750S i.e. The problem is that the peg (or hook) is too short. You can put a riser (or washer or whatever) on the lower part of the seat and it'll work. I'm all thumbs, so I had my dealer to it for me. Works well now.
I am really glad to hear this since I recently ended up with Scotts old seat. Nice to know it will stay put.What did they change on the newer style Monsters?
On the older Monsters the Corbin seats fit just fine and never come unlatched.