The easiest and closest to factory fiber padding for the bottom cushion in the rear is to get some sleeping bag mats (K-Marts are best) -- one of the 3/8 closed cell foam ones and two of the egg-crate type (Coleman brand is good).

Remove seat, remove cover (expect to sew a new listing on the back, the fabric "tail" there will likely be rotten along with the padding).

Cut the thin closed cell foam pad to fit the springs and sew it down with some cotton string and some "picking out" needles from the hobby shop. The needles are 2 and 3" long. Use some cotton string as thread.

You can use one or two layers of the egg-crate foam -- I don't remember exactly, but I think I used one. Align the rear edge to the rear of the springs, points down, then cut to fit the front with enough left over to fold back under, around the front edge of the springs, leaving about 1" sticking out. A bit more won't hurt. The reason for this is that the original padding is larger than the spring cage. You can measure the cover if you want to be exact -- the piping on the cover should be in the center of the edge you roll over.

Fold the foam over and glue it with spray adhesive, then sew it in place. Use some thin cardboard or heavy paper on top and bottom if you have trouble with the string cutting through, and pull only tight enough to hold the foam in place. Sew around the sides and back to keep the foam from slipping around.

Takes an evening or two, but it will sit much like the factory padding for about $25.

And replace the leaking window gaskets, front and rear, that caused the padding to rot in the first place.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!