This style combo-lock is better than the spin type as far as ease of use.
And no guys, the combo to my shed is NOT my address!
Usually you have a bolt on the opposite side you can move to lock it.
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From the front I would just use a combo lock. That way you don't have to worry about key.
We use these at work and they last years outside.
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Just spray some penetrating oil in it every few years. Also when it is unlocked. Spray some oil down in to the mechanism.
I am on ~4 burn years with these:
They stay outside year round too.
I like the idea! But I'd recommend a combination lock. If you got a Master 175D Amazon Link here you could just write combinations on pieces of paper. It has 10,000 possible combinations, so you won't even come close to people guessing it. Also there are a few other benefits: 1. This eliminates the need to buy extra keys. 2. You don't have to try to collect the keys 3. You can change the combination after each lotto (which prevents someone from making copies of the winning key and mysteriously winning each time!).
I know that box isn't prepped for a padlock, so you could either attach a hasp or get a locksmith to install a CompX EcoForce. Link here. It is a mechanical cam lock that should fit that door.
Source: I used to be a locksmith, now I wholesale locksmith supplies.