An Aprilaire 600M humidifier attached to my HVAC vent is leaking very slowly, about 2 ounces per day. I have not seen it leak but the leak is coming out near the exhaust at the bottom of the plastic insert (see first pic below).

This is what I have tried.

1. Inspect the housing for cracks
2. Make sure the plastic insert is seated properly
3. Pour water into into plastic insert to see if it is leaking
4. Pour water into the pipe where it exits the humidifier to be sure the exit piping is not clogged.
5. Change the water panel

I noticed some white crumbly stuff at the bottom of the humidifier (see second pic below)

What else can I do to prevent the leak?

Thanks.

UPDATE

1. I opened the humidifier cover and turned up the humidistat so that I could see what was happening when the water flowed. I saw water dripping at the bottom corner, originating about 1/2 up the side. (see third pic below)
2. I slipped in a thin wooden shim so the panel was not leaning against the holder (the tray) and the drip stopped. (See fourth pic below.)

I do not see how to put in the panel so it does not lean against the tray. Any help would be appreciated.

[![Plastic insert (view from bottom)](https://i.stack.imgur.com/3O6Bs.jpg)](https://i.stack.imgur.com/3O6Bs.jpg)

[![Powder in humidifier](https://i.stack.imgur.com/Sa6IU.jpg)](https://i.stack.imgur.com/Sa6IU.jpg)

[![leak](https://i.stack.imgur.com/DFOvV.jpg)](https://i.stack.imgur.com/DFOvV.jpg)

[![enter image description here](https://i.stack.imgur.com/Ul7ZP.jpg)](https://i.stack.imgur.com/Ul7ZP.jpg)