This is first take on the exterior idea with LOW POLY model. I have explored proportion and volumes for a while.
DETAILED 3D SKETCH
Once I got confident enough with my proportion study I started more detailed sketching in 3D. Much like black tape on clay I made a network of refined curves which bacame guiding structure for my exterior model.
Finally projected surface onto my 3D sketch. After few takes and changes I was able to refine the model and have it mature enough for reviews, mill, printing or viz.