I need help turning the attached model of a shoe sole into a solid. Currently, it is made up of various surfaces. Ultimately, I would like to use the Cavity tool or a similar feature tool to turn my sole design into a mold. So essentially I need a negative of my design. Can anyone please help me with this?

Current steps I've taken to try to resolve the issue is to heal gaps and use the Thicken feature to turn the surfaces into solids but I have not been successful enough for it to be selected in the Cavity feature. 

Any help or insight would be greatly appreciated!

-Eric