I got to my local good will and pick at least 2 t-shirts, both the same size.
Then take 1 of them and cut it at the point where the extension is gonna be.
Then cut a chunk out of the other t-shirt to fill that extension and to the size you would like.
Put both pieces inside-out and pin them together along the ledge.
From there use a sewing machine and sew the 2 piece together.
once done, remove the pins, turn right side out and put on.
Thanks and have fun :)