An upholstry shop might be able to custom cut you a sheet of foam for that seat. Foam rubber sheet comes in different densities and thickness so you might be cautious of that if trying to match the original.
I am not a trucker but an office worker and a habitual road tripper.
I have the deluxe purple seat cushion. I didn’t think it made a difference at first but I can definitely tell when it’s not there.
I won’t say it supports the back, but it does make my work out car seat and office chair more comfortable.