Is it okay to use a sleeve that doesn't fit a laptop? (extra inch of space one way)?

I recently had to buy a new laptop that is smaller than my old one, so I thought that instead of buying a new backpack and a sleeve with a strap, I just bought a laptop sleeve that I liked for the new one on ebags.com to add some extra protection to my new laptop. Ebags.com has a system for matching laptops to sleeves/bags that has the dimensions of laptops to match up, and I used this system, but the sleeve ended up being too big width-wise (like up and down across the keyboard/computer), which is the way the laptop goes into the sleeve on the side. There is about an extra inch of space there. It also feels like the laptop does not fit very tightly height-wise, although it fits almost perfectly (just a teenie-tiny bit of room that makes it easy to slide in and out, so not much at all) lengthwise (left and right across the keyboard/computer).

So is the sleeve definitely too big and should I try to return it and find a new one? Or would it be possible to find or make some sort of insert? I'm skeptical about making or finding an insert because the sleeve is one of those flimsy ones made out of wet suit material. I've tried cutting up a piece of styrofoam that came in the sleeve, but it keeps being moved to the side because of the flexibility of the sleeve. This is also the first time I've ever bought a sleeve like this since I like higher-quality material, but this was just for a little extra padding to keep the laptop from slipping around in the larger 17 inch bags I have. (The new laptop is about 14")