Rattling Heat Shield Beneath Car


03.5 Protege5
Yellow '03 5-Speed MP5
Two of the nuts (or plastic pieces) that the bolts from the exhaust heat shield screw into have stripped out or broken off inside the floor of the car. I'm not able to tighten them or loosen them since they just spin freely. So naturally, the heat shield rattles like an MFer. Does anyone have any ideas how I can secure the heat shield so it doesn't make so much damn noise?