Behind seat panel

[cgster]

Does anyone know of a panel that I can use to put behind the seat to take the place where the gas tank originally was?
I didn't know if they make some kind of cardboard that fits in that location. I would just leave it as it, but the back of my cab has a lot of dings and Id rather just cover them up. Not looking for carpet since im going back with a rubber mat flooring. Thanks.