Here’s the model of the firing tube.  The large tube is the right diameter for a Ping-Pong ball.  The holes in the sides are where the pitching wheels will go.  The bit off the side is where the motor mounting will attach.

This is printing right now, and it says it’ll take about an hour and a half to finish.  I tend to have bad luck with prints like this.  They always seem to peel away from the baseplate or split at one of the layers.  It if doesn’t work, I’ll try using PLA, which is less like to do that.  Unfortunately, PLA is more likely to gum up my printer…

The motor fits fine, but it’ll slide right out if I don’t secure it using the screw holes.  So I’ll need to stick them in the model.
The slide rail, however…  The slider is way too loose.  I’ll need to shrink the hole just a bit.

I don’t know how far apart the wheels will need to be.  So instead of printing out something with exact (and incorrect) measurements, why not make something adjustable?  This will let the motors slide, then get locked into place when I find the right spacing.

The 3D printed tube is a perfect match.  Time to get working on the other components.

The cardboard tube didn’t cut it.  So why not just make a tube to my specifications?  What else did I buy a 3D printer for?