Construction of the mold form
1) doesn't leak
2) keeps the original part from moving while molding
3) minimizes the amount of molding material required, while keeping enough structure to keep the original form while casting.
The mold form will depend on the object to be cast's shape, and the molding material. Thickness of the walls and "top" will need to be adjusted as best fit. The base of mold form is the top of the mold. In this example 0.5" walls and bottom would be ok for plaster.
The mold form needs to be sealed about the seams so the casting material will not leak.
Notice in this example with the "L" box construction the over lapping seams help to seal and hold the mold with glue or clamps.
This can be made of MDF, Cardboard, etc. Calking, Hotmelt, or sculpy can be used to seal the form. This also applies to the seam around the base. In some cases found objects, legos, 3D printed forms, cut bottles or thick aluminium cooking pans can be used and formed to make the mold forms.
insert table of recommended wall thicknesses.