At least one brand, and I've forgotten which, uses a specially molded (roughly) triangular shaped plastic fairing at the forward edge of the awning case to make it blend into the Sprinter body with a more finished look. I think they also use one at the rear. If someone could figure out how to get that part it would solve your problem. I would be interested in one just for the aesthetics. Your suggested solution should work as well. Let us know if you do your own fabrication. Anyone know which manufacturer uses that fairing?
Current: 2018 Midwest Automotive Weekender MD2 (Sprinter 3500). Previous: 1990 Prevost bus conv. 2007 Fleetwood Rev, 2000 MCI bus, 2014 Itasca Cambria