Most Electrathons do not have suspension.  Many tracks are rough (race in the NW, some very rough).  An easy way to make a lightweight seat is to take a plastic trash bag.  Spray some spray foam in it (you will need to decide how much you need).  Close the bag and set it in the floorboard of the car.  Sit in it for a while.  You will have a very light weight, sort of comfortable custom molded seat for your racer.

We use a stadium seat cushion.  Works pretty good.
One race we forgot it and a student borrowed a pillow from the motel.  It was a little  bulky but worked O.K.

Vic

Most of our cars make a hammock seat. We use a nylon mesh (fine mesh about like screen doors) and then just suspend them from a removable bar that the top and bottom of the seat. Your car can bounce all it wants and you don't feel the jar. With the seatbelt tight you are still very stable in the car so have great control. It works great for my 56 year old body.

Our cars all have solid seat bottoms and backs, so I use 1/2" thick foam sleeping bag pads cut to fit for padding. They are made of closed-cell foam, so never completely collapse... Mike, I may steal your mesh idea on my next build. Sounds like it would be a welcome improvement to my 61-year-old bones.