As you've found, it's quite tricky to get the screws in exactly the right distance to make the power strip nice and firm. If it's on a wooden wall, or there's a convenient wooden batten behind the strip you can place a single screw above the middle of it, pointing down at about 45 degrees, screw it down until the head of the screw nicely locks it in place. If you've got brick walls, then fasten a wooden batten directly across the top of it, fasten the batten with two screws and wall plugs - or perhaps even one?.