What sort of aircraft do you have in mind for the pitot installation - light or heavy? You'll probably be aware that there are a couple of wing-mounted pitot head designs: the 'L' shape that you'll find on Cessnas, for example, and the straight ones with a 45 degree (ish) impact face as fitted to Pipers. The heads themselves are attached to the lower skin via screws to enable the units to be easily removable (replacement of heating elements is the most common).
I think you'll find it's common practice to have the doubler fitted internally, as with aerials. Keeps the airflow tidy and is better cosmetically.