I took a shot at fixing that with netfab & LCD_Cover_Solid.stl could not be fixed properly with that. I am not using the latest netfabb though. The problem with that STL file seems to be the text he added into the middle of the model. I added it to the Slic3r Prusa Edition 1.36.2 to see that. This slic3r seems to do a reasonable job of slicing it without doing a repair on it, but that text in the middle does not look like it will be readable. That LCD_Cover_Solid.stl I would rotate x180 or y180 before slicing it. Have you tried the Prusa version of Slic3r?