You could try what I do--stop the mixer, add the flour, take a piece of parchment and wrap it around the bowl as a cover, then start the mixer and let it run for 20 seconds or so, after which flour spills are unlikely. (I picked up this tip in an online class, if that had been the only thing I learned from that class it would have been worth the class fee.)
Why parchment instead of a towel? Well, if for some reason the mixer blade catches the parchment, it just rips. If it were to catch the towel, it'd drag it into the bowl. I've never had it happen, though.