Dynamic Alpha Gradient Texture Collision Detection

I'm using about six different alpha gradient textures on-screen --each of them using a simple quad.

Problem is I want to raycast hit-detect the object I'm clicking at but the results can be very erratic because the "mesh collider" using quad's default mesh doesn't take into account the alpha gradient.

Is there any way to work around this? Even as a GUI texture possibly? Although I don't know if it's possible to raycast hit-detect a GUI texture.