How to make a flower box:
How do you assemble a wooden flower box? For a large flower box it is better to make a double bottom. That way you only have to fill one third of the box with potting soil. This saves a lot of soil and the flower box will be a lot less heavy. It is best to make the planter from impregnated wood.

First make two wooden side walls of the same size.
Saw six beams at the height of the side walls.
Place a beam in the middle of both side walls. Do the same at the edges, with the bars placed slightly inwards.
Then place two sides on top. Now you have a square/rectangle.
Put reinforcement strips in there, which you fasten to the joists. The top slats are placed at a height of about two-thirds.
Place the (double) bottom on the strips.
Cover the inside of the tray with a piece of agricultural plastic and fix it with a staple gun.
Staple the plastic about two centimetres below the edge.
Cut off the remaining plastic.
Fill the flower box with a layer of hydro grains and potting soil and plant it with beautiful plants or flowers.