Gave it a try last night...however no pics, and not a lot of progress a bit chilly in the garage. Apologies for a long post...
I found that in order for the "Turkey carver" to work well I had to approach the cut at a perpendicular angle, then turn into the carving once into the foam.
I took the old seat foam remains, laid them flat on the new foam, drew around it with a sharpie marker. I left everything approx. 3/8" bigger than the mark so it would fill the covers. Plus because the old foam had powdered I had to guess a little.
Began the cut square to the foam and worked around the rounded edge.
Very much like cutting wood, start from a square shape, then cut down corners to create filets. Used three inch HD foam for the bolsters, then layered on 1/2 foam across the face.
The only real problem I encountered was gluing the new bolsters to the old seat back because of the powder remnants. I had to literaly wire brush the old stuff off to get to solid material for the glue to adhere.
I used 3M spray adhesive. Sprayed the touching sides on both the old and new, and squeezed them together.
I have the drivers seat back glued up, complete with new burlap backing @ 2.99 a yard. It does not look like professionally built foam, but once in the upholstery, it should fill it out nicely.
As far as the webbing goes, so far the Spit's have been fine, but on my MGB I used tie down strap. 1 1/2" wide nylon strap like that used in cheap ratchet straps. Cut to length plus 3 inches. Once the end is wrapped through the wire hooks, I used a hot awl (16 penny nail) to melt two holes and used 2 rivets with washers, rather than sewing. Just as strong as a line of thread, and easier for me to accomplish in the garage. YMMV. I just cut the old webbing off the existing hooks and reused the hooks. But I imagine that they could be made as well, just think miniature coat hangers...