Sealing small cabinets

I just purchased a few counter top humi's... used in gas stations, bars, etc... made from mahogany. They seem to be put together well, save for the back, which is maybe a 1/4 inch piece of wood that isn't sealed fully.

Hoping to get some idea's on what would be best to use to seal the cracks... should I use silicone, find other pieces of wood, etc...


Thanks for looking.

Dan