This is probably a method used by a few people, but I think it's worth sharing anyway just incase - specifically because of the awkward blurring areas that make this game an unnecessary pain. I've used this method for the last year or so, and it's pretty good when it comes to making masks accurately and quickly. It also works for facepaints as well.
Step One: Import your grid
The grid is the key. It makes it clear where your lines need to go, where your logos will blur, and where they