Start on a 20 meter circle (either direction at any gait). When you hit the long side of the arena, half pass to center line.
More advanced versions:
Steepness of the line, add zig zag in the half pass.
While a 10 meter circle will set you up for half pass, a 20 meter circle will test if you can change the degree of bend to match the line and how well can you adhere to riding on the line as well without compromise.