Yes, it's crude but at least I can refine the shape and length and while I know it sits properly by drawing on the cardboard. On the ROTJ helmet the back or the rear "cheesegrater" sits a little higher than the ANH one. I had some pics where that was evident, put they are somewhere on my server. Strangely, I can't find any pictures of the ROTJ filming helmets lately to show you...just fan made replicas.
Since this version of helmet was designed from the ground up in 3D and I didn't use photogrametry, the shell took a lot less sanding than the old one did. A layer of photo resin hides all the joints and layer lines.
I picked up several thicknesses of 4x8 sheets HIPS several years ago at a plastics wholesaler. I used 0.06" (1/16) thickness for both my ANH and ROTJ versions.