To make mold

Hi,
just 2 simple questions:
if I want to prind au mold (example for fishing lures), must I fill the support (cube) before Boolean modifier of the project in my software (Blender 2.80) to have a good result or can I let it like it appears on my screen ? In the second case, will my mold be good for molding or not after printing ?
Thanks for help.
Regards
Yves