Thats all I'd do, use the cardboard. Maybe string if it was bigger. Main thing will be keeping the to pieces straight wild welding. Unless that insert pipe is a tight fit. If it is it doesn't matter if its not perfectly straight as you can fill the gap. I'd probably put a mild bevel on the two pieces and leave a gap so I could get in there and weld it to the insert pipe for both pieces then just fill it up.
A3 sheet of paper will make it around and as long as the ends are parallel you can tape it on and trace a line. Might need some second opinions on welding it though, filler, technique etc. Gonna be a fair bit of load on it.
Nowadays people know the price of everything and the value of nothing... Oscar Wilde