I use a similar setup as your stock tank except mine is galvanized.  We knew we had cold, Oregon winter weather so my husband cut out 2 pieces of plywood the shape of both ends of the stock tank.  Then he nailed a lip onto the outside of the plywood making a lid.  These 2 pieces are cut so that the heat lamp hangs down in between the plywood leaving an opening in the top big enough just for the heat lamp.  I will post a pic when I get home tonight.  I don't think a wire top will keep enough heat in for a 32 degree night.
	
		
			
		
		
	
				
			 
	 
 
		 
	 
 
		 
	 
	 
 
		 
 
		 
 
		 
	 
	 
 
		 
 
		 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		