Rail fits via the 4 clamp points by the ejection port/magwell. Battery space is admittedly pretty tight but it fits (barely lol). If you plan on getting stuff for the handguard I'd recommend either cutting the screws down or buying shorter ones. I used 8mm M4 screws to stop any puncture issues. it's the 5KU Monolithic Rail.
Installation is identical to this:
EDIT: Sorry forgot to show battery space with all the shit attached: