Because it's going to be hard to keep the cross upright and straight for a glue with a slow setting/working time, I would use a fast setting C/A glue available in small tubes at HD or Lowes - Dap makes one that I use that sets up in 30 seconds. Put a dab on a piece of plain paper and use a toothpick to apply a small amount to both the cross and the building and hold in place steadily for 30 seconds. I would first do a couple of dry practice runs to make sure everything fits together properly before gluing.
You can also buy and apply a C/A accelerator (spray or brush on) with one hand while you're holding the cross in place with the other hand, that will give you an almost instantaneous hold.