Toss your internal set screws and replace them with proper bolts.  You can tighten them till the collar splits, and it will, and not strip the head out. 

I use the solder and washers, actually I drill and punch tin stock not washers, on my CL planes.  It is lighter and won't come off during flight.  Reheat the solder and it comes off.  Fly a decent pattern in CL and have a wheel come off on landing and you've lost your flight.

You 'CAN' file a flat spot where the wheel collar locks down.  Probably doesn't hurt anything as most of us never change the size of wheels on your airplane and many will wear out wire gear before they wear out an airplane.  I've not really seen that be necessary. The  only time I've been bit by wheel collars is when I've relied on that terrible little set screw that uses a cheap tool of not precise size to fasten the screw with.  You never know if you get the screw tight or did the wrench slip or did you loosen the screw when you had to wrench the wrench free.

Replace the set screws and you will be happy.  I've used Dubro or Midwest or anything I had lying around.  If it was from a Chinese kit and I had a fitting swap I've used that too.