S.B.
I don't know if this is what you're asking or not. I swaddle my little boy in his car seat with a blanket. I just take the blanket I use to swaddle him normally, and lay it in his car seat. I set him in on top of the blanket, then buckle him, and then I swaddle him by wrapping the edges of the blanket around him and tucking them under. That way he's buckled properly, and also comfortably swaddled at the same time. It keeps him calm.