Thanks for all your comment!
Inspired by Ray's comment, I prepared two bags of silica gel using the same soup stock bag used as the air filter. Then wrapped around the air filter using rubber bands. The opening of the silica gel bag was stapled closely. Once the silica gel absorbed moisture the whole bag can be dried in an oven (100 - 120 degrees)
It works!
In addition, I made a dehumidifier to attach EP folder to keep inside OTA moisture free (while keep in storage). Jaycar sells a sealed ABS box with a clear rid (a changing colour of silica gel can be seen without opening the box).
All works very well at a fringe of cost. (Except I needed to invest a hole saw for the exact size of the PVC tube... thought around $25
).