I started looking for a sewing pattern to use -- not much out there unless you'd like to recycle a belt and then THAT pattern seems to have made it's way in various forms all across the internet. Trust me. I also found loads of covers for existing straps. Not what I needed either. I finally found a pattern for knitting and felting a belt (in one of my favorite Half Price Books finds: Knitting With Balls) that was exactly what I needed.
Essentially, used Cascade 220 wool and a width of 3 inches prior to felting for the strap. I measured the boy with the guitar to figure out what length it had to be (about 36 inches). I told myself I should write this stuff down and I forgot to, but I think I used size 13 needles and 9 stitches, but as they say, you should check your gauge before you begin. I use a garter stitch with two strands of wool together. I love the ridged pattern it produced! I wasn't sure how long to go, and I tell ya, 44 inches was a bit too long -- you can see the tail there sticking out from under the guitar. The wool didn't shrink in length very much during the felting process. (I probably should have followed the advice to knit and felt a swatch ahead of time, but really? I was WAY to eager to start the project.)