I mounted a Burris FastFire red dot on my 858 using the scout mount. Burris sells a metal protector for the FastFire which attaches to the sight with a couple of screws. The sight/protector unit then mounts on the rail of the scout mount using a single large thumbscrew. The whole thing works like a charm.
As an aside, I also ground off the portion of the scout mount rail that projects forward over the handguard. This allows easy removal of the handguard for cleaning access, without the need to remove the sight.