Long Drywall screws might work. They are case hardened and will thread into stovepipe so they might thread into metal fence posts. A fender washer might help in keeping the crews from going through the insulator.
If that does not work, you might try long pop rivets.
Drywall screws won't last long in the elements, look for something made for outdoors, coated or plated.
Look for self drilling screws. They have an auger type nose that will drill through light metal but they're a bit pricey. If the metal is thick it could dull the point before breaking through. Self tapping screws are hardened and the threads will cut into metal. Cheaper but you will need to drill a pilot hole to get the point started.