What to do?!?!?!?!?! If only there was a pattern available to make one! So we searched and found this (free!) one at Sew Mama Sew.
Em and I worked on it together. We improved the pattern by adding a front pocket and doubling the interfacing to add extra cushioning.
The pattern includes the formula to determine the size for any laptop. We wanted the end result to fit the dimensions of hers, but be just big enough to hold the PC snugly. Since we added the extra layer of interfacing, we had to tweak the formula. Hence the experiment!
We first made a "test" model of just the outside front and back using muslin. When we slipped the PC in, it was already snug. Uh-oh! So we made a second version a 1/2" wider. This seemed better, but without the double-interfacing and lining fabric, there was still a significant margin for error that hung over us as we began the real case.
Though the directions were pretty clear (and had excellent accompanying photos), we made a few mistakes along the way (like using the wrong color thread). So we both got practice in ripping! But nothing that we couldn't easily recover from.
Finally we were done!!! We put the PC in! The side dimensions were right on, but it was sticking out just a tad on the top. So we ripped the bottom seam and resewed it at 1/4" instead of 1/2".
Here is the result: