Here are some samples of cracked and badly repaired early type Twin Cam air filter backing plates. It strikes me that even with good welding ground smooth the cracks are destined to reappear again with more vibration. A proper production solution would have been to use thicker steel. My suggested field fix here would be to add a second layer of sheet metal, about 1/4 to 1/2-inch smaller than the original perimiter, all holes same, and solder the to pieces together to double the thickness.