Australians have been using a rag known as a Chux wipe in their boxes.  The bees start to shred the rag, and it turns it into a fluffball.  The beetles try to hide in the rag, and the hooks on their legs trap them in the rag.

http://i279.photobucket.com/albums/kk123/ozbee_photos/SANY0024_zps0d100c8d.jpg
http://i279.photobucket.com/albums/kk123/ozbee_photos/SANY0023_zps58af6098.jpg

You could try some kind of fuzzy rag or cotton balls or maybe steel wool above the inner cover since the bees won't be going up there and shredding a rag.

A CD case can also be used as a beetle trap.  That will fit above an inner cover too.

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