Great Idea, I look forward to seeing the end result !
Well the lower half of that thing is all wires and tentacles :
https://www.dakkadakka.com/gallery/912299-Vivisector%20Cult%20%3A%20Magos%20Deum.html https://www.dakkadakka.com/gallery/910459-.html?m=2 I used
GS cable maker, electrical wires of different sizes, and metal strings.
In order to place them into shape, I started forming the metal wires, that will stay into shape on their own, then attched the electrical wires and plastic ones, glued them to the metal ones so that the thing stays in place. Last were the greenstuff wires, that I mostly used in order to fill the most obvious gaps in the structure.
It can also help to tie all the cables together on one end, tie them together with metal wiring, then heavily glue them in PVA. This way you have a strong spot that lets you shape the rest of the thing.
I also relied on the base in order to maintain stuff together. And after the basecoat spray, went for a second round of greentuff filling, as the paint made obvious some areas that would be impossible to paint.
Have fun !