4) here i've cut out the paper template, it's slightly longer on one edge so that it overlaps. i've cut holes out of the keel line on the template so that i can see the keel line on the tube. tape over these bits first.

when you mitre both ends of a tube, you need to know the ends are aligned, and the correct distance apart. The keel lines allow me to align my templates, and a reference line on the template gives me something to measure to. here i've extended the top cutout to include a small part of the 'ref' line - so i can see the keel line, and the ref line together.