I went kind of crazy with my ruler and some cardboard to make this planting jib. The thick lines are 9 per sqf and the thin lines are 16 per sqf. I just simply stuck a pencil in the corresponding hole to mark out all the planting positions. When the garden is in and I have more time I will make some wooden ones.
Here they are all planted out 9 per sqf using the jig. It worked pretty well and certainly speed the process up. The pencil made a perfect hole that the onion seedlings were planted right into. In total I planted out 84 onions in the photo above as well as 13 multiplier onions in another area. Here are all the varieties I am trying:
Utah Sweet Spanish Onions from seed shared by Hometown Seeds
Red Long of Tropea Onions from seed shared by Daphne