I found what seems to be the same product on eBay, $1.96, free shipping, and you can order one at a time. Downside is, eBay sends your name, address, and presumbly payment information to China, and the item ships from there. But I'm okay with that, at least in this case, and mine arrived in less than 2 weeks. Here's a link: -- and searching on eBay for some combination of the keywords "Kitchen Table Pad Tools Silicone Pot Holder Trivet Mat" should get you more options, if you wish.
The squares are just slightly less than .308" across, on the inside. Here's a pic showing 3 SNS Hy-Tech coated bullets sized to .309", standing in the mat next to each other. Notice how the outside edge of the outside square is bowed out slightly by the base of that bullet. Doing the math, the outer edge should be bowed out for about 3x the amount by which one .309" bullet pushes one edge outward, so you can see the .30 cal's are a light press fit into these squares. (I tried to measure inside the squares with a caliper, but they're just too dang flexible ... )