Did you cut a relief cut to invert the head? If you didn't, you will never fully succeed in getting all the flesh removed. when using the real skull, you need to cut the skull just above the jaw muscles to open up the brain cavity. You will need to scrapes away all the meat, and replace that the day of mounting with apoxie sculpt or clay.
The best advice has already been given. Use an artificial head, or cut the real bill away as you would if you were going to use an artificial head. If you fleshed properly, then you must have a flesher that will aid in the removal of the meat that is attached to the skull. With a Canvasback, you will definitely have shrinkage on the bill, but it's not out of the question to use the real deal.
I hope this helps