Here are a few pics of my original backseat bottom being dissected.
First one just shows what it looks like from the bottom before I go at it. Things of note on the OEM construction to clarify what you can and can't replicate. There is no single piece of burlap by itself covering the springs, it is incorporated into the first layer of sewn batting. So (and rambow correct in any instance I am wrong) we replace that with a sheet of burlap, covered by a sheet of felt. Also on this bottom seat, the batting stops right at the edge of the seat, the sides and front are just stretched vinyl seat cover like a tent. They seemed to neatly trap about 1" (compressed) of batting between the perimeter of the seat cover and the frame, not sure how they did that so neatly.
Next we see a wire reinforced plastic sheet that lays directly over the springs but stops about 6-8" short of the ends ... why I have no idea. Saw this on both bottom and top seats.
Next we see the combination burlap/sewn-batting which does extend all the way to the seat edges where it is hog ringed in place. Note the sewn seams running lengthwise through this layer.
Next we see the top of the previous layer, rolled back you can see the top layer of batting that is directly under the seat cover.
Here is an edge shot showing the layers and their relative thickness after 50 years of compression.
