So I replicated the support pieces and tried some different angles to see if I could space the building correctly.
The next step was making the big round thing in the back. I used an old plastic christmas decoration and cardboard. I painted the cardboard with a few layers of watered down pva to make it very solid indeed.
The next step involved gluing on some random bits from the bit box, squeezing in some Green stuff to close the gaps en more amounts of watered down PVA to seal the building.
For the next step I covered the building in glue and sand, made a base and forgot to take pictures.
After applying some paint, some weathering powders and a freehand I'm very satisfied with, this is the end result: