By Robert Raucher and Rich Wisniewski
Robert Raucher: Bend two thin 1/4 inch length strips of aluminum in to an "L" shape. Aluminum strip is maybe 1/8 inch wide. Drop one or two into the hole carefully, with the SHORT side of the "L" resting flat on the body of the car. Then carefully re-thread the Tenax fastener into the "smaller" diameter hole and tighten.
Rich Wisniewski: I cut some galvanized sheet metal about 1"x1" and then drill a hole in the center of each, the correct size. Check the size by inserting the Tenax fastener on the bench to assure it will be nice and tight. Once satisfied, drill out the rest. Then get into the wheel wells and hold them in place while someone inserts the Tenax in place from the top. Make sure you also use the plastic washer so as not to mark up the paint. If you want you can then spray some flat black paint to hide and protect the metal.
Gene Choin suggests using "pop nuts" or "jack nuts" inserted into the holes, then insert the Tenax. There are various sizes of pop nuts available, just pick the one right for the hole you are working with. Same sites have German and metric, so go whatever way you want. I've been happy with the SAE (English). Sources here:
OEM Fastening Systems
Byler Rivet Supply
Brad Ripley at NLA notes that Tenax fasteners Made in England are different than the genuine German Tenax. The shape is different and the plating is nickel, not chrome. For the true German stuff, see this PDF file www.happich.com/pdf/Tenax.pdf.