I think his problem as he states, is the tires are too close to the trailer frame, nothing to do with fenders.
If your having to get rims, you probably can pick up a set with a different offset to make up the difference your looking for, while forgoing the cost and worries of a spacer.
(That said, I have run aluminum spacers with the lugs built in from Trail Sport, they make them in any pattern, without any noticeable increase in bearing/unit bearing wear after several thousand miles on a rig.)
I'd go with the wheel offset first, if your looking for that little bit of clearance.
J.R.