need help with an Optical Illusion

Sand it.

Probably impossible.
1. Perception of edge sharpness appears to be a hard-wired function of neural circuitry (see Lateral Inhibition)
2. Edge perception is also function of background field (what is not the box) over which you have no control.

Make the image stand out more than the canvas (box).
A spiral or porthole-view, view from a framed-circle (cave entrance), The sun.
Aviod straight edges and symetrical-focus.