You don't have to worry about the extra 3 mm spacing that many keirin frames now put into their rear spacing to accommodate tugs. Just let the flat part of the tug sit on the outside, directly under the track nut. It isn't quite as non-slippable as having that part on the inside, but you'd only see the difference in UCI-level sprinting or kilo.
That being said, the easier solution is to spread the stay ends a hair. You can do it with your bare hands on most bikes (try it in very small increments because you'd be surprised how much you can over-spread it). If you can't budget it that way, grab a car tire jack and use it instead. Jack it open just a bit and leave it for an hour. You'll find the metal retains a bit of a memory of its new position. Again, work in small doses.