Google result for upholstery filling.
 

Coir fiber coconut upholstery fiber filling horsehair substitute.
Used in traditional upholstery as a stuffing, can be used with horse hair, or as a cheaper animal friendly alternative. Coir fiber is the husk of the coconut, this makes for an incredibly versatile & springy upholstery filling.

Is that easy to form into those molded rubberized mats?

Crubling Seat Buns
 

Yes, German vehicles used coconut fibers, older American vehicles used horse hair .

The image posted clearly illustrates why I always mention covering the bare springs with muslin , canvas or something before putting the buns, used or new on them .

My problem had been : the thin cloth at the unseen edges of the seat covers is dry rotted and needs to be addressed else the cover will come loose again soon .

I don't have a sewing machine nor the skills to replace the crispy delicate cloth .

I _do_ have some very nice covers harvested I want to try to place over my old unbroken spring boxes .

As promised here are the images of my rear seat respring. It worked well. I just wired in some springs along the front edge of the seat then replaced the covers. Cured the sag. I removed all my pool noodles.

Before:
https://live.staticflickr.com/65535/...718cb4d860.jpg

The fix:
https://live.staticflickr.com/65535/...48f7793a10.jpg

detail
https://live.staticflickr.com/65535/...d661b436dc.jpg

all back together with much better contour. no more sags.
https://live.staticflickr.com/65535/...8a53b0825a.jpg

Thanx, looks good .

Nice work!

It’s great seeing people restoring their seats.
Too often you see someone dealing with rust, a bumper, a paint chip, or say, the red of their seat belt buckle receiver, meanwhile living with totally bombed out interiors.

After vacillating for over a year, I had my backseats pleats restored by a local upholsterer.
It’s like a new car now.