I agree with Al Needham, hardware cloth is the way to go.  I use 1/2" mesh,
but some feel a mouse can get through that and recommend 3/8".  (I've never
had a problem with a mouse getting through the 1/2".)  I used to fold and
insert it as Al suggests, but now have changed that.
 
I now staple the hardware cloth to a piece of wood that is the width of the
entrance and approximately 1/3" thick.  I cut up scrap for this purpose.  I
leave about 1" of hardware cloth below the wood, and then staple the wood
the  hive body so that the hardware cloth covers the entrance.  The skunks
used to pull out my hardware cloth, but apparently can't get enough leverage
to pull out the staples.  I leave the hardware cloth on year-round.
 
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