Foam is a great tool to make molds. It is cheap and easy to machine
Here are some examples for foam molds
Concrete molds
Lost foam castings
Custom retaining walls molds


1LB EPS foam used to mold a 6' concrete shape






EPS foam molds - concrete was poured around
And when dried - removed to make windows

Tapered EPS foam concrete mold

Tapered EPS foam concrete mold cut with a hot wire

Tapered EPS foam concrete mold cut with a hot wire