So, I had to make some sculptures for an artistic project and that's what came to my mind...
Step 1: First Make the Base
Make a box with some chipboard, mine was 40 cm per side, and 10 cm high, and wrapped everything with stretch wrap.
I added a grid for screed, cut to size, for greater strength.
I also made a guide for the fixing screws, so it's easier to slide them in.
Next mix the concrete with water and pour it in the boxes.
Let it dry, open the box and put the base aside.
Step 2: Second Make the Sculpure
Next make the structure for the mold.
My lay-figure didn't have the head and the legs so for the head I'd use this tutorial
and for the legs a couple of polypropylene tubes wrapped with paper.
Then wrap everything with plastic bags so it won't stick.
After that wrap all the structure with two layers of stretch wrap and PVA glue for outdoor use.
Let it dry, it could take long...
Meanwhile make the internal structure with a wood pile and a crutch.
Cut the mold from the feet to the head and put it on the wooden structure.
Fill the mold with stretch wrap rolled up or polystyrene or bubble wrap or anything you have.
Drill some holes on the base of the wooden structure to fit the fixing screws.
Close everything with scotch tape and more PVA glue.