@yourusername- You can do six if you do one at the most extreme spot in a top corner of the element, then one to the side and one below perfectly lined up with the outside edge. Then you repeat in the opposite corner.
It would be nice if there was something to indicate upgrades were available seeing as you have to scroll to see some. Even without, a really nice game.
That sounds like a good small feature to add this weekend - thanks for the suggestion!