I made two caps (using an adjusted version of this newsboy cap pattern ) in the appropriate colors of red and green. Then, I made two tiny mustaches, cut from a sheet of black felt. And, of course, Mario and Luigi would be nowhere without a magic mushroom or two, so I knit up a little plush power-up (using this pattern as a guide).
They love the caps and all the accessories, and I love looking at them wearing them. They're the cutest plumbers ever.