I used a grey hand towel for the backing and based my measurements for the fabric strips on that. They ended up being 11.5 x 4.75" each, which gave me a couple of inches to spare and made it easier to center the zigzag.
I used an old towel for batting to give it a little more weight, and quilted around all of the seams like he suggested in the mug rug tutorial. I had originally planned to sew right sides together and turn it (like in this tutorial), but it was too thick to try that, so I ended up binding it with store-bought quilt-binding.
Then, thinking that hand-sewing anything on a something you stand on at the bathroom sink was a little cooky, I tried sewing the bias tape down with the machine. After one section I realized that was going to look like crud (as you can see in the picture above), so I took it out and hand-finished the binding after all. A little fancy for a bathmat, but the colors are perfect.
The fabric is from Amy Butler's Nigella line, home décor weight, in passion vine "stone" and "celery." My mom and I found it at Valley Fabrics in Stuarts Draft for just $6.50 a yard last year and we bought it all up between the two of us. Sadly, we've never seen Amy Butler or any of my other favorite fabrics there since.
Showers of happiness to Kate and Nick!