Felted Laptop Cover

As a beginner I’m not sure about doing a gauge. Does anyone have a simple pattern for 17" Dell laptop cover? I have the wool yarn but do not know how many stitches to cast on. I want to felt this bag and know it must be much larger before the felting. Thank you from a beginner

Here’s a few. they may not all be 17", but might give you an idea how big to make one.