I poured some liquid latex into a form and waited for around half an hour for it to set.
Then I placed the test 3D printed piece in there (after putting a thin layer of vaseline on it for non stickyness) and added a extra layer of latex with a paint brush.
The mold is then built by adding a thin layer with a paint brush every 40-120 min.
I placed some cardboard around the 3D printed piece to make the concentrated point around the piece smaller.
Turns out that this process takes ages and this tiny mold took me 5 days to do.
When ready a thin layer under the piece inside the latex is removed and then the piece is removed.
Here it is ready for pouring!