Howell,I set castor mounds anywhere I want.I dont worry whether the beaver are making their own or not. If I find a huge natural mound I will usually set ti,when I say huge--I am talking a foot or 2 high.Those that are being used by many beaver and have been for a long time.
I also make my own near these large natural mounds.This can really infuriate the local beavers.
I sure am glad more beaver trappers are joining in.
Tom