This is a slightly more structured eyeglass case.
Because the item will be felted, gauge is not extrememly important. Anything from 4 stitches per inch to 5.5 stitches per inch will be fine. Ultimately, the case needs to be large enough when felted to hold your eyeglasses comfortably.

I used Cascade 220, but any worsted weight feltable yarn will work just fine.

Schematic and before & after felting photos included in pattern.