We play this song in my cover band. Honestly, those two instances where he uses the ray gun in the Rebel Yell solo happen so fast, it's probably not worth the attention you're putting into it. If you're playing covers, then like the rest of us, you're probably playing to a room full of a couple hundred drunks and only 1-2 of them (most likely musicians) would even notice if you nailed the ray gun part.
In my band, we play to tracks and I just have Steve's actual ray gun sampled and it pops in at the right time. But if you want another option that still sounds cool, just take your pick and scrape the edge of the pick back and forth really fast on the high E, B & G strings moving from the bridge toward the neck. Not quite the ray gun effect, but still sounds cool.