First thought it to get rid of the charger you have and any that are just like it.
Putting anything you think may catch on fire in a metal bucket is not a bad idea. Make a cord with a GFCI. The GFCI should trip is anything goes wrong and shorts to the bucket. May put a ground wire on the metal bucket. Perhaps a metal cover on the bucket and place the bucket on bricks.
What kind of maintainer are you using?
I have my mom's old electric wheelchair, with Ni-CD batteries. They discharge on their own. So I have a mechanical timer that turns the charger on for 30 minutes/day. Seems to keep them up.
I also use a timer on my dewalt charger. One hour is more than enough to charger a battery. That way I'm not cooking the battery.
It could be your battery causing the problem too. Try a timer and a smart charger. Juice the battery for 30 minutes or so a day.
Personally, I don't leave a trickle charger on 24/7.
I purchased a batteryminder 12248. Directions say to charge the battery and then let it sit for a few weeks or more. I rotate batteryminder to different batteries.
Santa is bringing me a battery conductivity meter. Hope to get it soon. This will tell me where the battery is in it's life cycle.