Thanks to sneakypoo for this awesome design!
The head is constructed for three filter layers á 1cm. That seemed like an overkill and I only wanted to use two filters. But as you can see in a lot of makes of this fume extractor: If you use less than three filters they do not align perfectly and look wavy or bumpy.
So I came up with this very simple redesign of the optional backstop. It hosts a 1cm high spacer ring. This way, two filters stack on top of each other and neatly align with the rest of the head. No waves, no bumps. 🙂
download the files on: http://www.thingiverse.com/thing:1964393