Not exactly hive covers, but more on plastic frames:

Interesting explanations regarding why bees may not "respect" bee  
space as well with plastic frames.  If the cause is either reduced  
distance between cells and space (narrower frame) or the impression  
that plastic is wax, then why do bees not brace plastic frames on the  
sides?  Could it be that bees sense tension or propensity to sag on  
comb and brace accordingly?  Plastic frames may be a bit "plastic",  
especially when loaded.  Some top bar hive designs have sloped sides  
to reduce comb attachment.  The idea is that the angled comb is  
perceived as structurally stable, not requiring bracing.

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