For the KAC panels to lock in place, the rail must be notched to accept the locking clip. The MI handguards and the KAC handguards are notched to do this. I am currently evaluating the rifle length version of the MI handguards, and I can tell you , the KAC panels DO NOT MOVE. They lock into place and become part of the rail. The TD rail covers I have do move slightly, fore and aft, but its not bad. So if you get a rail system desinged to use the KAC panels, they will lock in place and you will not have a problem.
I'm not sure if any other rail systems are notched that way or not. I know the YHM I had was not so notched.
You can always secure the panels with a section of inner tube to hold it on the rails a little tighter.
But I like the KAC rail covers, if the rail is set up for them, the TD rail covers for most other things, and the MagPuls I'll have next week to play with.