I have two diaper bags, even though I mostly use the one, I have a smaller one that I can take short trips to the store and a larger one that I can take on outings that might last a while. And between small samples and such, I have enough of most things to make both bags fully stocked. The only thing that I thought might be essential that I did not have two of was a changing pad. So I made one.
I had some vinyl tablecloth material I bought for a variety of uses when I was pregnant. I purchased a yard, and then cut that yard into two fat quarter and had a half-yard piece left over. The half yard I believe will go in my trunk for times I might need a piece of material that is waterproof. One of the other pieces I have left as is, and the other I made into my second changing pad.
I decided to use a cloth diaper as the backing, but you could use any fabric you wanted. I also decided a bit of padding would be nice, since I might need to lay the baby on hard surfaces. So I cut batting to size, and held everything together wrong sides together. I did not pin it because that would make holes in the fabric, rendering the waterproof-ness void. I also did not quilt the pieces together, which would have been nice to keep it more stable, but that also would have left holes. I thought about quilting just the batting and backing together before sewing them to the vinyl, but that was after the fact. Either way would work if you tried this.
I the stitched along the edge, leaving a space to tun it right-side-out. I clipped the corners and any excess before turning, turned it all right ways, then topped stitch along the edges.
It definitely is not my best work, but after looking at her modeling on it, I realized it will be needed when she gets too big to fit the one that came with the bag!