Making a combination slide bolt

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It's certainly sturdy, but with only 3 digits in the combination it couldn't really be called strong. A slide bolt like that would be on the inside, though, so an outside attacker wouldn't be able to access the dials until they go over the fence, however at that point it may well be simpler to just unscrew the device from the gate. 

It may be good enough, though. Mostly it's going to slow people trying to take heavy objects (ones that can't go over the fence the same way the people got in) off your property. No lock is undefeatable but if it slows them down enough that may just provide enough time for police to arrive, or it may simply demonstrate that your property isn't the easy pickings they thought it was and convince them to try some place else.

At least there's not a huge gap they can see through

from here (image source)

It seems the incentives relating to getting security/privacy right in this bathroom are as misaligned as the sliding bolt hardware.

Fuckin' slide bolts, how do they work?

from here and here (image source)

When you don't know how security works, not only are you likely to use it wrong, you're likely to install it wrong too.