A Spaceship For Joe by Doug Irvin
A short tale
Joe has a problem. It's summer vacation and all his friends are either away or down with measles. His Uncle Joe suggests that he build himself a clubhouse. He opts for a spaceship, instead, out of plywood.
Melissa, the younger sister of the boy with measles, comes by to tell him that he can talk with her brother Billy at the window -- she has to do that herself -- and to look at the spaceship. That's when its appearance precipites them into adventure.
Spaceships, interstellar law, and fractions all feature.