I made my first trash can mold using a heavy cardboard tube, the kind large-format printing paper rolls come on. It is at least 1/8 in. wall thickness maybe 3/16. It was just under 3" i.d. as I recall. I had a good supply of them so I used a bandsaw to cut them to about 5 or 6 inches length. I used some kind of conical object (that was shaped so it would come out) as the core to make the cavity, might have been a solid nose plug from a 105MM howtizer projectile. I used another piece of cardboard tube to make a spacer that centered the core inside the bottom of the tube section. I sat the tube on a steel plate around the core and spacer ring and poured hot lead into the top of the tube. There was no real top piece to the "mold" I just filled the tube to the top and topped it off slightly as the lead shrank down a bit on cooling. There was lots of very stinky smoke, don't do it inside the house; garage might work. The cardbard is charred but stays intact just long enough. You peel the cardboard off after the stuff cools. Believe it or not, they worked fairly well! The rounds look a bit odd, flat noses, slightly raised spiral mark around the outside where the tube seam was, but they worked! You could very easily make a top piece that fits inside the top of the tube, with a hole in it to pour lead in. This, if made with a "stop" so it always seats the same depth on the tube, would give better consistency to the finished size.
The next one I made used a steel artillery cartridge case, I think 76mm, which we sawed off at top and bottom to take the diameter we wanted. We then sawed a slit along the side and welded rectangular bar stock along each side of the slit so we had a raised surface where we could clamp the two sides together with a C clamp. This was needed in order to remove the finished casting. The cartridge case steel was "springy" so we could open up the sides a little to remove the casting. We made a lathe-turned cone for the core with an increased diameter at bottom serving to locate and center the mold around the core. The top plug had two diameters separated by a shoulder that centered it in the top, and had a pour hole in it. That mold worked better and didn't smoke. The mold wall thickness was only about 1/16 inch, if that, so it certainly isn't correct for a mold which should be much thicker, but it worked well for something we threw together in a few hours.
I still have the mold but it isn't at home so I can't take a pic of it now.