Here are photos and measurements of the accessory phone mat that Honda sells to go on the shelf under the shift buttons. It fits so well that I had to use a knife to get under the edge to remove it. I also made a full-size scan jpeg. Caution: If you're going to make your own mat using the scan, note that the edges of the mat are beveled, so the scan will appear larger than the actual bottom of the shelf. You can see the bevel in the 2nd to last photo. Finally, to demonstrate that the plastic mat is pretty hard, the final photo shows the maximum angle at which my phone will stay in place without sliding. The mat could have been grippier, but then I imagine it would have eventually become this hard after years of baking in the sun.