I have a method that works well for me, but it does involve using 1 commercial product, TurboCAD Deluxe. I have version 21, which is not the latest. I open the STL file in TurboCAD, select the object, then explode it. If there are multiple parts in the model, I sometimes have to explode more than once. Any flat surfaces are turned into polylines. If you explode too many levels, you will end up with individual lines & not continuous polylines. I save that file as a DWG or DXF & open it in DraftSight. Set my view to Front, Back, Left or Right & delete all the lines showing up vertical. I then delete horizontal lines that would be duplicated when viewing from plan view. I usually have to orbit view around the model some before deleting any horizontal lines to make sure I am not deleting what I need. Depending on how complicated the part is, this usually only takes me a few minutes.