I am just getting at trying a mouth call so am in no way in the position to give advice of mine but here is what I've heard are some differant options forgetting apurr. Try (1) fluttering the lips like you mention (2) get that motorboat sound to from back in the throat (Some people can dothis and some can't) or (3) instead of fluttering the lips flutter the tongue, it feels like it's fluttering against the roof of your mounth and the diaphram