If your opening is like many Cobras, there is a lip on the fiberglass around the opening. I would use something like this in a smaller version and attach the expanded metal to this clamp. Then you can tighten the clamp on the fiberglass lip. It would be a bit of a pain initially, but after you got he clamps in location, you could bolt in the expanded metal.
https://www.amazon.com/6Pcs304-Stain...s%2C80&sr=8-41