The usual way to make thick, even very thick, lines is to just make a pen extremely fat. The downside of this approach is that the line ends will be rounded, not square.
Another way for a very fat line - like wall thickness thick - but with square ends is to use the fill tool. But that's too hard, you say! The trick is to set the Wall Tool to the thickness of the desired line and to draw your polyline with the wall tool (set to basic wall - one fill - the fill and color of your desired "line"). Make sure Grouping is "suspended" - and then select these 'walls' and explode them, clearing the checkbox that would keep the original wall. You'll be left with 2D fills and 2D lines (wall edges).
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