My battery box covers also don't fit tight but I'm sure glad I still use them. Earlier this year one of my two standard lead acid batteries (Interstates from Costco) developed an internal short during our trip to San Diego. I didn't realize it until the next morning when I happened to go into the front basement. My IR thermometer had that battery at ~200º and it was off-gassing
There was a little smell of gas in the basement, just enough to make me notice and check the batteries, most of it vented to the outside. You could see the gas rising off the battery.
Bottom line is that I'll keep using my battery boxes, even with AGMs.