FFF printing optimised mask holder

Update 2020-04-17: added V3 of Holder for improved stackability. Mind: some printers may not like this model since it has minor bed adhesion.

This mask is based on the EDAGmask4all design (https://www.edag.com/en/edag-group/press/corona-crisis-team/edagmask4all) and was remodelled to be better printable on FFF style printers. It was designed for the MakerVsVirus hub Munich face mask initiative.

Changes to the original EDAG design are as follows:
- removal of the EDAG logo and lower height part to improve printing speed and allow for better stackability of the parts
- enlargement of the blocker hole by 10mm to cater for larger faces
- reduction of the holder hole by 0.2mm on all sides for a tighter fit of the blocker (we use one layer filter cloth, the original clearance was too big for that)
- enlargement of the strap holes since the original holes tend to glue together (are too tight)
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I print these files with 3 perimeters, 0% infill, 4-6 bottom and top layers, layer h