Help with front wheel hub motor installation

I have previously built two electric bikes on cheap (and heavy) steel frame bikes without issues, and they work very well. I am trying to install a front hub motor on a lighter (but steel frame) bike and I am having problems with the motor popping out of the front fork dropouts whenever I hit the throttle. I have tried tightening the nuts and using lock washers, but if I tighten too much the force literally squeezes the wheel out of the dropouts (no doubt because the dropouts are open on one side so the force is applied asymetrically). Has anyone else had this problem? I have ordered a torque arm which should help but I wish I could really tighten down on the nuts on washers that tighten the wheel onto the dropouts. Any ideas?