Those inner sprung blitz seats are just the best!
you bounce along.
It is like riding in an FJ Holden. No jarring on your back.
I did a job for a mattress factory that burnt down last year and they made a mattress that used 6 inch springs that would be perfect to make a seat base out of.
It would be very easy to make and possibly cheaper than new foam.
Have you bought any lately?
Just my 2 cents.(too young for pennies)
Cheers
Jeff