I hate to tell you that, but your chances of the healing are very low. Duck legs don't mend that well at all. There is either expensive surgery by the vet, amputation by the vet in the hope that duck learns to deal with, or putting the duck down. In either case a splint is very less likely to work. I wish I had better news. See if you can find a duck rescue organization or a rehabilitator who is willing to work with you. Or better see if a vet is willing to work out some payment arrangement with you. Don't let the duck suffer. The duck also needs some antibiotics for the bite.