Found some pearl clay from a pound shop, and it's absolutely great stuff! Easy to shape and attach, plus it dries quick. It might not stick well to completely flat surfaces, but works just fine in nooks & corners. I'll propably paint these little pearly eggs pink.
For burnt umber (=darker) parts I left the toilet paper out of the mix, but instead drenched sheets of paper in the mix to create kinds of roots or veins. Shaping and molding them was a bit tricky, so next time I'll try to use more glue and paper to thicken up the mix.
Thick goo mixed from PVA, paint, toilet paper, fine sand and a few drops of dirty water. The burnt sienna coloured version of the goo is just the components mixed togehter and painted on with an old brush.
Spackle was used to blend things together. Think I applied it with just fingers and shaped with some stick to match the surface textures of the seashells. Definitely nodding towards H. R. Giger, Tyranid organisms and Nurglesque lifestyle here.
Mixed some PVA, paint (black, green and browns propably), fine sand and dirty water. Tried to avoid the more detailed bits and concentrated on covering the polystyrene parts, in case I decided to spray paint those pieces later.
Polystyrene offcuts were used to make concave support pieces for overhanging platforms. I'll cover them with cables or organic looking junk later. Another tip: use (for example) two dice to support the cutter on the table, and slide polystyrene sheets through it to cut "perfectly" level platforms!