I don't think I'm gonna add the strap, but maybe, the material is upholstery vinyl from Joanne's fabric . and the angle is because the fire wall is leaned back some so its to makes the master sit more levelVery nice work on the adapter plate. Why is there a downward angle towards the front of the car?
Also, nice work in the panel recovering job. What material did you use? Are you going to add the strap to the upper door panel?