I've added a loop and button fastening to my small texture felt bag. I hand spun some thick yarn in the same colours as the bag, using merino and some 'fancy fibres', then I plyed it and felted it by wetting, soaping and rubbing.
When it was dry, I chose the part I wanted to use then positioned it on the bag and fixed it in place by sewing with regular thread. I then covered the sewing with a piece of felt and blanket stitched it into place. The button is one I hand made from fimo a while ago.