I've tried wooden, plastic, and masonite inner covers and I don't care
for any of them.
 
The inner cover provides a haven for ants and the extra activity in
removing the cover provides an aggravation both to the bees and the
beekeeper.
 
Unless I find something cheaper, I use plywood (I really like BB
formboard grade), tack and glue about one-half inch of regular wood on
each edge and use cleats on the ends.  I also cover mine with offset
plates from the print shop.  What I have told you also identifies myself
as a part-time small operator, retired and not concerned about the extra
time.  I am aware that large operators would not do it this way.
 
Jim Shoemaker
5160 S. Western
Brookline, MO 65619