Dusts can creep into the E-mount interface and they are then be sucked to the sensor:
And these dusts are micro, cannot be blown off with a blower. You must use alcohol to clean them up.
I found this O ring from a local hardware store for about $1:
It is about 45mm x 2mm, perfect to seal the E-mount slot:
Works for a6000, a5100, a5000 and all E-mount cameras including A7 series. No problem for lens swap. May keep water out too, to some extent.
If you cannot find such an O ring, you may use a proper rub band, but might not be so friendly to lens swapping: