"Do you not worry about them having their front feet full of mud and leaves when they approach the mound from the water?"
I've had them do it both ways. Some will simply charge the mound and tear it up and squirt their own scent on the torn up mound, and others will approach the mound with their front feet full of mud and leaves to cover up the castor smell I placed on the mound. I reckon it depends on the beaver.
If they are approaching the mound with mud, you still have a good chance of a hind foot catch with the trap being positioned as KY shows. If they are simply charging the mound, you have a good shot at either front or hind foot.