My wife and I have found that a piece of plexiglass works great.  You can
get it at Home Depot or Lowes type stores.  Measure the door and get a
piece that is an inch wider than the door frame so that it will be snug
when you place it there.  This works best on door that have a door stop
or molding that you can put the vertical edge of the plexiglass into,
which keeps it from slipping.  I normally get a piece that is 24 inches
tall.  We have tried shorter, but one of ours could jump up and climb
over the edge.
 
If you are concerned about the possibility of cutting yourself, I have
found that putting electric tape on the edge of the plexiglass works out
very well.  Plus, if you put it on the vertical edges there will be less
risk of damaging the doorway.
 
--
Chuck Steel
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http://www.steelfarms.org/chuck
[Posted in FML issue 3803]