Thats exactly what I did H. Hover a tube over a hole in the ground and just start pouring. I used quick set concrete. That helps stop concrete spilling every where when you start on the tube, but dont muck around, do it all in one go, fast is the key
. You dont need reo and all that palava, just remember to stick threaded rod in the top for the mount before it dries
. This is the cheapest way to get a base and pier, and its all in one go. The tube can be what ever doesnt mater, its temporary. I used wrapped gal sheet. Cardboard would do.