The Ruffwear bag does look good. It may not be too hard to draw up a pattern if you have an eye for details, but I imagine it will take you about four times as long as you think (that's at least how things have gone for my MYOG projects).
Maybe I'll try this soon, though, with something tough and light like DP VX-21. It's a great idea, and something that would only lightly affect my hiking if I have to carry my dog's stuff. It's not like she couldn't come along until it's made.