Helpful to mention what year. From your avatar the Early Ford V8 Club 35-36 restoration book has what you want.
I'm curious as to where you are sourcing authentic material fabric from?
Sorry for the delayed response... I was on another project.
RE: 'I think the next question will be: How do you know how much excess to leave, along the outside of the plastic, to accommodate the material and padding? Did you wrap the material over the sheet or fold it tack it on the outside of the plastic?'
I wrapped the material over the sheet and folded it back on the backside which covers the corrugated plastic sheeting edges very well.
I did not have any templates to work from at all so I just placed some heavy card stock cardboard against the inside of the doors and sketched and cut until I had a workable template. Each door took a couple of tries but I soon had what I needed. This, I then used as my template for marking up the corrugated plastic sheeting. That sheeting was easy to cut with a sharp box cutter. I placed the the corrugated plastic sheeting onto a plywood base up on my work table and just cut slowly after marking it up with a Sharpie marker.
Another trick I used was after marking all of the spots for the door panel clips in each of the corrugated plastic sheeting panels... (I chose to use nylon clips vs the original metal clips...) and placed them thru each corrugated plastic sheeting panel hole. I then used toolbox drawer liner foam rubber over the corrugated plastic sheeting which covered the nylon clip heads very well so they would be well hidden and not shown thru the fabric covering. This foam rubber sheeting is between the corrugated plastic sheeting and the fabric material covering. It also added another layer of insulation and sound proofing.
All this to say that about two years out it has held up very well for me.
There has got to be a reason I am missing about not using a very thin Masonite or Luan type material. Is it moisture? I'm in need of all four door panels form my 42 Mercury and was planning on the same process. Cut a heavy Kraft paper template, presume the two doors are mirror images of each other, so one template for front and one for rear. Drill handle holes and drill clip holes, cover with fabric and thin foam padding for a nice soft cushion, wrap around the door panel and glue the backside all the way around. Trim strips would be drilled through the fabric and board and bent over to hold them in place.
But, in my case, I've got to figure out the door handle mounting.