Chad and others,
One of the members of our local MBCA chapter, an enterprising and energetic 16 year old, did this to his 300 turbodiesel sedan with astoundingly good results. They could not move around as there was no place for them to go.
I think the thing to do is to understand that you should probably repair the steel basket first, either by repairing or replacing the broken spring steel pieces. I can almost guarantee that on any 35 year old car, at least the driver's seat needs work.
Second, if your "horsehair pads" are broken down they should be replaced as well.
Finally--to augment the seat and add support--the empty spaces can be filled with pool noodles, cut and or bent into the positions needed.
It is on my list of winter projects this year to remove the seats and repair the springs, and then add the noodles.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored