If you were using a pvc frame, a dowel of the same diameter would work. If you use something small, like an embroidery hoop, be sure to baste it well and take the hoop off as soon as possible to avoid stretching.
If the frame is wood, you may be able to glue or screw on a temporary splint, but I would go easy with a repair. A well placed clamp may be needed to reinforce the splinted section from below.