A small granary

The idea is to keep the grain well ventilated, dry, in the dark and safe from rodents.
You could consider adding aeration tubes to prevent over-heating and spoilage.

This is a small granary so it's structurally sound. If you make a bigger one, you need to add joices from one wall to the opposite one (for every sides) to avoid the walls from buckling outwards from the load.