I have the 6" and 4.125" panels and they fit pretty well. Just a very slight amount of fore and aft play. You have to install the side panels with the slit towards the chamber as the locking tab indexes on the first rail slot and the rail slots are asymmetric due to the barrel screw on one end of the rail. The side panel lines up with the rail only when installed as described. If you put it on with the slits and locking tab toward the muzzle, the first rail slot is farther back, and the rear of the cover extends past the end of the rail. On the bottom rail, the slit and locking tab can go toward the muzzle.
St least that's my experience on my SCAR.