The seat bases on my Spider had sagged quite a bit....... it was like sitting on a plank....... so I had a go at fixing them.
Because I'm a cheapskate, rather than buy new inserts, I managed to fix them by inserting some medium density foam into the "slots" in the original foam bases.
I didn't take any pics (sorry

) but if you remove the seats (4 bolts per seat) and then remove the seat base (drill out the pop rivets) you will probably find as I did that the original foam is still OK...... but the foam ribs have collapsed sideways.
All I did was cut some medium density foam such that it fits between the ribs and reassembled the seat.
The difference is amazing and as I said before it didn't cost much.
Well worth the effort