Never tried knitting them (just as well, as I can't knit) I have a few made from old acrylic fleece, jacket liners and sweatshirts - just a rectangle with a couple of holes for the front legs. Anything very small born on a very cold night gets one for a couple of days - just keeps in the lamb's own heat and, provided it has sufficient colostrum and the ewe mothers up well, it can be taken off after that and will thrive.