I was at my mother-in-laws for Christmas and my niece had made one for her son, who is exactly the same age as Matthew. "Well, I'm stealing the idea!", I told her. She told me she fond it on Pinterest, The next time Matthew was over with his carrier - I made one.
I went to the fabric store and found some batman material. I had to do a batman one and now we call it his bat cave. :)
Here's the link for the detailed instructions on how to make yours. It is nicely done and filled with a lot of step by step instructions. It's done by a young lady named Chelsea, and her site is Life With My Littles.
The only thing I did differently was put a piece of fleece in between the 2 pieces of flannel. It can't be too warm with our Michigan winter. And warm it was .... I used it in the car seat when he took a little catnap and when he woke up - he was sweaty.