Well the easiest way would surely be to use a grid with fixed locations.

But the second thought that came to me is an intersection of horizontal and vertical lines. So you just would need to have the fixed units at the borders.
The problem on that is that you would have to give units around to different players to check which actually are within the checked square AND that you cannot create units easily within a certain sqare, and that could be not comfortable...

... but would work

Wow, would be a great idea to make a tutorial of it.