I've seen all sorts of sheet material being used.  Linoleum flooring,
canvas, plastic.  But all reduce the beespace above the top bars, which the
bees normally use for movement between frames, and for evaporation of water
on hot days.

We use simple 3/4" plywood migratory covers with 1-1/2" end cleats
projecting downward, and also have end cleats on our boxes set 1/8" below
the migratory cover down cleat.  This allows us to pry up the cover.  If
frames also pull up, they bind on the box and break loose.  It's only
rarely that we need to go to special effort to free pulled up frames.

BTW, medium density overlay plywood hot tanked in paraffin lasts
essentially forever for hive covers.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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