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.

Print off a part and fit the hole with one M4 bolt, two washers and a wing nut.
Slice your sandpaper into appropriately-sized strips from the back and do any tearing necessary.
Wrap a slice of sandpaper around the holder and tuck the ends into the gap, then tighten up the wing nut while holding the bolt fixed.
Sand away!