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Mix together two parts sieved coir fibre, one part cement and one part sharp sand. Pour water into a well in the centre and mix it together to form a stiff paste.
Using a trowel, place a 2.5cm layer of concrete into the bottom of the larger cardboard box. Ensure the concrete is spread into the corners.
Cut a piece of small-gauge wire mesh to fit the bottom of the box. Place it on top of the mixture to reinforce the trough, and add a further 2.5cm layer of cement over the top of the mesh.
Push 5cm-long pieces of wooden dowel into the base of the trough for drainage. Then, place the smaller box inside the larger one, leaving a gap of around 5cm. Add bricks to weigh it down.
Insert sections of mesh along each side, cutting them shorter than the depth of the box so they'll be hidden.
Add cement to the sides of the mould to make a wall. Use a stick to push it into the corners. This will prevent air pockets from weakening the walls.
When all the concrete mixture is in place, support the outer sides of the box with more bricks. Then cover the entire thing with a plastic sheet.
After two days, when the concrete is set, remove the sheet of plastic and supporting bricks and peel the cardboard away from the sides.
The sides of the trough can be aged by roughly scraping them with a trowel. After a week, use a hammer and large nail to push the wooden dowels out of the trough, leaving holes for drainage.