Hi Stannum.
Yes i had considered the roughness causing micro vortexes, as the windows with hard edges would do the same, reducing drag at the rear on the rocket.. much as a Ute (or pickup in american) creates a pocket of swirling air near the rear of the car and reduced drag (as proven on Mythbuster), not sure if its the same on the rocket, but seem right:S... though i will probably taper windows for next build.
Maybe i should use two materials, one for the body and one for the fins and nose cone... Alumide for the body as it has a greater heat tolerance.
The main aim was to have it as cheap as possible, and use bare minimum materials, so anything i add will mean subtracting material elsewhere....
my original idea was to just have a nose and fins that connect to the engine and nothing joining the two, but that seemed difficult to design.