I would put another button on top (that is exactly the same), rotate to 45 degrees, and only activate the function when both buttons are pressed. This will make the active shape an octogon instead of a square, which is very close to a circle. here's a quick photoshop showing this theory:

See how they overlap? it's almost a circle.

If that doesn't work, you can make up your own button function using sin and cos to make a perfect circle as the detective radius.