9. Concrete Block Butterfly Wall
If you have leftover cinder blocks from a project, you already have the bones of a sturdy butterfly wall. The hollow cores become ready-made chambers.
Materials: three or four concrete blocks, gravel, bark strips, a flat capstone or paver, and a trowel. Stack the blocks in a staggered arrangement so the openings face outward at different angles. Fill the bottom half of each opening with gravel for drainage, then add a loose bundle of bark or a few bamboo lengths.
Set a flat paver across the top as a roof with a slight overhang. Because the blocks face different directions, butterflies can choose the warmest side as the sun moves across the yard.
Time: 30 minutes. Difficulty: beginner. This is heavy, so place it where it can stay permanently, ideally in a border near a shrub that provides additional cover.