Minecraft wiki entry for Survival_in_an_infinite_desert, associated text: 5×5 block sand walls, 3 blocks high, with empty door spaces 2-high in the middle of each wall (40 blocks total)
4×2 cacti for 'doors' (8 cacti, but more is good)
10 solid blocks (dirt is most portable) for the roof space (or 8 if you want to leave a 1×1 skylight)
4 solid blocks (e.g. dirt) for "lintels" above the doorway spaces (since sand fall down into the space)