In order to make forms sturdier,safer and easier to install I have filled them with aerated concrete and inserted the plastic tube in the middle. I intend to hammer a steal rod into the ground and slide the form on top. It is really important to consider and design installation systems while making forms, as it saves lots of hustle and retro-fitting later.
When they are ready for permanent installation I can glue the rod in the form, and concrete it under the ground level.
Here it is, at Gomboc Gallery and Sculpture Park: