don't.
just keep spinning, surface bs -> surface b2 off.
in all seriousness though, boxes are tough to lock because of their width. like the above posts mentioned, make sure you're popping into your 90 rotation, and straddling the box if possible. from there, your upper body will either set your rotation at 90, or continue farther if your hips allow it.
keep your eyes on the end of the box, as you would any other rail.