If you are just looking to firm it up,
stop past an upholstery shop and pick up some edgewire and bw clips.
You can run a length through the center (from side to side) or bend a rectanular grid in the center for more support.
Bend over the ends of the edgewire. Align the edgewire with the springs, so you can clamp them in a straight line with the bw clips.
This really will make a firmer seat.
We ordered new seat bottoms and backs for both our sons 65 and my 66 buckets. Year One supplied the 65 springs. Jim Dyer supplied the 66 springs.
Both were installed by myself and brought both seats back to original condition.
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