I''ll go with that type if I can''t find the flush ones. The Shakewell brand has a O-ringed plug that you slide up to open or push in to close. The hose connection comes off the side. Anyway, when you first install it, this plastic cylinder protrudes from the outside of the hull and you sand it off flush with the surrounding hull. All that''s visible when its closed is a thin circular line. The drag is probably so low it can''t me measured. But like I said above, I can''t seem to find a source anymore.
Kris