Extra fill inside to use less plastic

[Kingster]

I am casting some statues in a 1-part mold and because of the statue's size is pretty big I am thinking to throw in the mold a pice of another material, a random piece of it and after that fill it with the plastic that will go and fill the spots that are the details. The material I am speaking of will just occupy some space and will not be visible when I remove the mold (maybe just a little bit because it will "touch" the mold due to weight before I spill the plastic).
So the question is what material to use for it. I want a cheap material like clay! I am making cheap terrain molds for me, so I want to minimize the cost.

[Fleafa]

Buy filler designed for casting resins or consider rotational casting. Clay is not a good option. Waste cast resin or injection-moulded plastic sprues should be fine.

[Kingster]