The main "chassis" on this particular pen is quite thick and the inner dimension of the tube not so large. Medieval casting of a cannon sized actually, and needing a cannon for my crew I set to work cutting the plastic tube after removing the inner ink tube (that to be used to allow me to envelope with a flag thus allowing it to be slipped over an existing standard-bearer wire pole to have different flags for the same unit).
The cut tube was placed within a balsa wood frame, itself reinforced by additional bits to handle the cannon's recoil. Metal straps were of paper softened by a water and PVA(white glue) mixture. The whole was then primed and painted. A very quick, easy project which I "needed" to do.
While that is the medieval 'modern' version of artillery, they still had the old-fashioned weapon very much still in vogue (at least for a couple more years) in the shape of the catapult. Here is one I came across in the fantasy section. It comes already primed. While the wood is probably a bit rough and weathered for realism, I will be using it for objective purposes for scenarios (I don't have any figures suitable as crew at present).
Just piddling away at the many diverse projects I have on the go....