Step 3: build the tower
the dowel for the tower is half the length of the long members used for the base. in my case, that makes them 6 inches.
i placed the tower dowels 4 inches from the end and put screws up into it from underneath.
the support pieces (that make the triangular shape) are simply paint stirrers that i placed where i wanted them, marked where to cut, then used a set of wire cutters to cut through them to shape. then i put them on using staples. these supports are very important, since they transfer the torque put on the tower dowels by the catapult to the base.
i drilled and screwed screws into the tops of the tower dowels. the tower dowels can be spread apart easily (which helps when installing a new rubber band). that's ok, because the only force applied to the pipe (the rubber band) is orthogonal to the pipe (a 90 degree pull - that is, it pulls away from the pipe - not along its length).
next, i simply held the tower dowels apart and inserted the pipe -- the screws hold it in place. the hot glue gun glue keeps the pipe from wiggling too much, but doesn't secure the pipe.