Assuming that the thing has some readonable length [which you do not mention], just wrap a sheet of paper around it. … a strip from a sheet of A3 would be good.

Provided that you have one good straight edge to the sheet, it's a simple matter to align it; add a strip of tape, and mark your tube with a permanent marker.

MichaelG.

If a flat surface with say a corner is available…..bring top of dome ( or cone or any “right” end..) into contact…measure from that “fence”..mark with scribe block or felt tip pen held at required distance..roll object against scribe……I have made that sound more complex than it is…

Paper method works quite well, another option is to stand an angle plate* on end and clamp the pipe to the inside as it effectively gives you a large Vee block. Then just use a height gauge to scribe the line around the job.

* needs to be one thats machined all over not cast on the inside