I'm sort of in the same bind - have a 1/5th scale Cub with 5/32" axles and the Du-bro wheel is 3/16" and yes, it is a tight fit!

I do what has worked in the past. Take an Allen Wrench and cut off the majority of the shorter 'arm' - just enough to get into the hex bolt ( that is holding the collar on).

Be sure you file a nice notch in the axle for the hex bolt to slide into :-) I also like to use a thread locker + some thin CA to make sure the collar stays where it is!!!

What kit/plane are you working on?

Jerry