mikeh,
That is an "Excellent" tip.
I just went down into the garage to inspect my hot water heater vent panel and sure enough, it was exactly as you described. 90 seconds later it is now as you advised.
Mine was that way for 14 years, hanging on by a tiny bit of a crimp. Now it will never come off without me first bending back the 4 tabs.
I can see how you lost yours after adding your very nice insect screen that happens to introduce outward pressure.
These simple tips are awesome. Thank You!
Ron Dittmer