A simple IK rig or spline IK could probably work to get the basic deformation and then maybe some corrective blendshapes, probably driven by the joint angles to fix any minor issues that might look off.
"I want the strap to stretch when the case reaches the maximun aperture and to bend inside the case when it’s closed." Looks like a spline IK with the ability to stretch would do exactly what you want to.
Hey guys! I Finally made something of this! Thank you! I used the IK Spline and two soft clusters to deform the curve but now I have a little problem, the base of the strap rotates and I want it to stay on place. How can I block the rotation there?
I really appreciate all your answers! I got a decent result with painting weights. I used the bottom joint to paint the weights because my IK spline is not moving there. But one last question. I didn't understood what you said with skining to a null object. It's like a standalone joint? I'm asking because I think thats…