Carseat inserts?

We got the chicco fit2 carseat for my son when he was a baby. Hes now out grown it and we loved it and want to use the same carseat for our second that's due in November, however we arent sure where we put the infant head support inserts when he grew past the stage of needing them. Can we use another brand of infant head support insert? We found this one that is shown on walmart with a chicco carseat so I'm assuming its okay? I put a picture of the fit2 carseat we have and then I put a picture of the insert we are considering... any moms have any advice or suggestions?