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Most snails prefer warmth and humidity - about 25-30 degrees C (or 77-86 degrees F) and 80-95% humidity. Research the breed of snail you are planning to raise to determine whether an extensive system is an option for you or not. Also consider wind as a factor in snail growth. Wind causes snails to dry out, so you should situate your snailery in a sheltered location if possible. Fine chicken wire mesh works well, because snails don't like to crawl on it. Concrete blocks and bricks are also a good option. Dig the fence in at least 20 cm to prevent snails from burrowing under and escaping. It only needs to be a few inches tall if there's no roof. If there is a roof, you'll need to build the pen at least as tall as the potential height of the largest plants in the enclosure. A roof can offer shade and protection from the elements, but it's an expensive option. If you have constructed your snailery out of a material that snails can climb, a roof is necessary to keep them in. Fine wire mesh is a good option for a roof. You can cover it with thin cloth for added protection. If you do add a roof, you will need a way to access your snails. If you are using wire mesh, attach the roof with twists of wire so that you can loosen them and get into the pen whenever you need to. Be sure that the soil is very loose. Snails lay their eggs in the soil, so it's important for them to be able to dig into it easily. Avoid very sandy soil (it doesn't hold water well) heavy, clay-like soil highly acidic soil (it can damage their shells) Shrubs and small trees will offer food and shelter. Plants that work particularly well are sweet potatoes, pumpkins, and leafy vegetables. You might also consider planting small trees outside of the enclosure as well. This will help protect them from wind, sun, and rain. Let it collect rainwater, since tap water often contains chemicals such as chlorine, which isn't good for snails. Use something shallow (like a jar lid) so that your snails don't topple in and drown. If the water starts to look particularly dirty or full of refuse, replace it.
Determine whether your climate is suitable for raising snails outdoors. Fence in an area based on how many snails you are planning to house. Decide whether you want a roof or not. Fill the enclosure with appropriate soil. Plant vegetation in the pen. Add a small container for water.