I used to have a mare that was a feeding freak too. No matter what sort of bucket I used she would throw the feed around. She was also one that would paw while she ate. The pawing never stopped but I ended up feeding her on a mat (4x8) and she never dug a hole but she always threw her feed. I would suggest having the horses teeth checked for a bad tooth.. that can sometimes cause them to be irritable during feeding.. sometimes not. ;o)
Good luck! If nothing else hire the little digger out as an earth mover!!