I got some sheets of sandpaper 230mmx280mm (a little wider and shorter than A4) and wanted a handy holder to turn strips of it into sanding blocks to make jobs easier. This is what I came up with.It is designed to grip sheets of sandpaper cut to 40mm wide by just over 200mm long, but with the files included it should be easy to re-design to your needs.
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Imported from Thingiverse: https://www.thingiverse.com/thing:9222
Original creator: 4ndy