@รอเร-อ 2 thoughts here....

How is the box attached to the timber?

Did you use the seal provided with the box?

From what I recall these boxes are cheap chinese with a string of rubber like sealant that goes into the groove around the cover and base. There 'should' 4 holes that allow you to attach the box to the timber that sit outside the seal. If however you've drilled through the back of the box to screw into the timber, the chances are that moisture has got in that way.

Looking at your picture the plastic seems to be degrading so the chances are the seals have too.

I've recently seem a product on sale in electrical suppliers that is a waterproof gel (a bit like silicon) that is poured or injected into outdoor electrical junction boxes. It's called "Electical Waterproof Gel". I don't know whether it's got any RF implications, but it supposed to be IP68 rated and safe up to 1000v. Might be worth investigating.

@keithellis Lovely idea.
I've followed suit and knocked up one but with the sensor facing the other way, so that it faces the meter, and the preset pot is and LEDs are facing away from the meter.

That way the sensor side of the case can be stuck to the face of the meter, and the pot adjusted without removing it from the meter. The flashing light on DO flashes in sync with the LED on the meter. Theres a little ring that holds the sensor flush with the face of the case so it can lie flat on the front of the meter.

Also, its a good idea to use black / dark coloured PLA so as to block out the any stray light that may illuminate the sensor (such as when the meter man comes to read it and shines a bright torch to see the dials).