Greenhouse V3 - Structure: Materials Nota
To optimise the number of battens that you need, and to minimise the amount of wasted wood, the following dimensions are a good compromise (all dimensions are measured inside the greenhouse, see here): For 2m40 battens: WxDxH = 1.10m x 1.10m x 1.34m. For 2m50 battens: WxDxH = 1.16m x 1.16m x 1.40m. In both cases, you will need 11 battens.
Inside the box, you can either use geotextile or plastic sheets for "drainage" (google). In french, search for "feutre géotextile" and "drainage vertical alvéolé" (example). If the greenhouse will be put on a terrace, a hard surface, or polluted ground, you have to cover the bottom of the box with geotextile that let's the water through. If the greenhouse is put on soil, you only have to protect the sides of the box.