If this is physically impossible to some people, I would suggest that they try looking at some of the artists from whom Magic Eye was derived.
Try these artists:
Victor Vasarely- http://www.vasarely.org/virtual-gallery.html
Bridget Riley- http://www.allposters.com/GetThumb.asp?CID=AF9687DA85C14FB6BC0D2AB5EAF49789&c=a&search=Riley%2C+Bridget
And Homage to the square (specifically the studies with more extreme colors) by Josef Albers- http://www.allposters.com/GetThumb.asp?CID=AF9687DA85C14FB6BC0D2AB5EAF49789&txtSearch=homage+to+the+square