Do a search "pool noodle seat" and you 'll get a variety of approaches.

Basically i took one of those pool noodles, The long floating noodle things, cut off about a 1 foot section lengthwise and cut about 1/3 off one side horizontally and stuffed it between the top and bottom of the spring, such that it is horizontal, i.e. parallel to the ground.

It was just enough to firm up the seat, on both sides, while still retaining the feel of a W123 seat.

Cost $1.50

JCD