So, after a lot of research and thinking, I'm planning to do the cowl all over again :
- remove the canva I put earlier
- sand the cheeks shapes
- add some vinyl material (same as a snowtrooper but not glossy, I hope I can find it) above the mouth area and cut the top vents after
- glue it above the cheeks too
- use the same technic as the snowtrooper (ie one vinyl sheet on the front and one on the back)*
* @IcyTrooper I now the cowl on the ref material is in one piece but I think this is a matter of animation/rendering as I really don't see how to manage it without at least a seam. What do you think ?