You need a long flexible driver and (preferably) small hands. Attach a socket that's the correct size for the bolt head to the flexible driver, part fill the socket with blue tack or similar and insert the bolt head into the socket. Reach around underneath with your left hand until you are near the bolt hole in the rocker cover, insert the flexible drive/bolt from the top with your right hand and guide it into the hole with your left hand.
Tighten it down and hope you never have to do it again!!!
Other forum members may have better methods.