… if you could find a metal funnel, you could heat the tube and then press it down on the metal funnel while the plastic was still soft and deformable. Otherwise, some kind of cone shape would work. An inverted funnel is what came more readily to mind for that.
A method that I used to flare out the end of a PVC pipe was to heat up the end of the pipe over a gas cooker flame until the pipe was pliable, and then quickly press it down onto a glass bottle (essentially funnel shaped on top). Gloves were necessary as the pipe gets quite hot. I was concerned at the time that the glass bottle would be too cold, and in turn cool down the pipe before it flared out sufficiently, so I filled the glass bottle with hot water. I am not sure now that it was really necessary to use the hot water, but it certainly didn’t hurt either. Additionally, once I had obtained the flare I quickly dipped that end of the pipe into cold water to set the flare in place, as the pipe seemed to try to return somewhat to its original shape if allowed to cool slowly.