You have to be wasteful I am afraid.
If you look at the old cantrail material you will get the idea. It has lots of v-cuts where it has to fold around the back of the cantrail.
Once laid flat, the material needed to cover a cantrail is much wider than a cantrail.
Lay the cantrail on the material and mark the shape. Make sure you cut too much so you can bend the cloth around it so you can glue the material to the back of the cantrail as well.
Mark your glue lines with chalk. Once you start to go in the wrong direction you have had it. You will miss the end of the cantrail woith the material. For that reason some people glue the top length of the cantrail to a large rectangle of material before trimming it at all.
Glue all along the top length first. Then deal with the wrap arounds, making V cuts as you go to remove excess material on the corners.
It is not too bad a job.