I received a package in the mail in a large bubble envelope and realized that it looked the same size as my laptop. I tried it out, and it just fit. I covered the envelope with duct tape and extended the flap as it was a bit short and I wanted it to overlap. I added some velcro closures and it was done!
Just FYI: this project took a whole roll of Duck brand Duct Tape plus about a quarter of another roll. I think each roll is 10 yards, so it took about 12 yards, maybe as much as 12 1/2.
I know it's probably not the most protective laptop sleeve, but it works for me. I'm sure it won't last forever, but then again what does?