About 3 years ago I have sewn a cover for a infant car seat. I still had some of that quilted fabric left over, so I decided to use it for a diaper bag. The fabric is already so colorful, I had a hard time combining it with any other fabric I own, so I opted in the end for some denim (from an older jumper that I had cut up). I didn't have a pattern for this, I just made it up as I was sewing. I think maybe it's a bit boxy and the proportions are maybe not the greatest, but it's functional. The only thing that is still missing is some kind of stabilizing piece of plastic insert for the bottom of the bag. I am still figuring out what to use.
The front of the bag has a pocket that closes with velcro (button is decorative), the sides have one pocket each and in the back is a flat large pocket also with velcro. There are two pockets inside. I didn't have enough length for the shoulder strap, so I made a kind of shoulder pad with the contrasting fabric.
I had a bit fabric left over, so I used a piece of plastic waterproof sheet and bound both of them with some FOE to make a changing pad.
And here are all three matching items waiting for baby to arrive - car seat, diaper bag and changing pad: