When I added my boxcollider to the Waypoint block, I made it only 10,1,10 instead of 10,10,10. With a 10,10,10 box, if an enemy block was on the camera side of my friendly block, I was able to detect the boxcollider while hovering my mouse over the enemy block. By making the friendly boxcollider 10,1,10 means that I only detect OnMouseOver when my mouse is over the top of the friendly block, ie. closer to where I want to place my cannon.
