A castor mound is a pile of mud and debis off the bottom about the size of a double handful up to a half gallon. Often multiple deposit over time will create a much larger pile. Beaver deposit scent in it to mark their territory. Often find them on points, always close to the water.
6 inch wide channel is a bit narrow, maybe twice that width and up sounds more like a beaver channel.
Swim through traps or scented cage traps probably closest to cheap and easy but I wouldn't categorize them as either. Conibears probably easiest but you would need a special permit for that.
