Diagramless crossword hints (Year 9 Variety Puzzle #21)

Outside the Box Puzzles Year 9 Variety puzzle #21 is a 17x20 diagramless crossword. Some solvers prefer to know what type of symmetry (if any) the grid has when solving a diagramless. Some prefer to know which square 1-across starts in. If you'd like either or both of those hints to get you started, here they are behind spoiler tags.

Symmetry hint: The grid has left-right mirror symmetry.

Starting square hint: 1-across starts in the fourth square in the top row.