One alternative I have used on previois blasters was gluing strips of sintra (sheet plastic) to the barrel to simulate the fins. I suppose the best would be a centimeter (3/8" for you yanks) strip of 2mm sintra topped with a smaller strip of 3 mm sintra creating an inverted "T" shape.
Of course, the preferred thing is to abandon the Fett project, get a DC-15 (aka a real man's gun) and some clone armor and defend the Republic. Then again, I may be biased. 8)