It's an 18x18-inch square of stout, unhemmed, waterproofed cotton, and for compact and minimal loads, it's the best carrier on Earth.
When you get a flat tire, lay it out on the ground like a place mat.  All your tools are there, and won't get lost.  When you're finished, wrap them up like a burrito, and secure it to your seat rails or seat stays, using a toe strap or an Irish strap.  You can't make it yourself for this cheap.  You can stuff it in a big bag's pocket, too.  
 
Creamy green, unwaxed, but very water resistant.

Now also available in Waxed Brown Canvas.