STL to Surface 3D Conversion

I need to convert the 3D file of this figure from STL to a watertight surface file. Also need to find a parting line to split parts for molding, eliminate undercuts, and shell the resulting files to a thickness of 0.4mm. Up to 4 shell pieces total is acceptable. Some loss of small details is allowable. The file must be usable in STEP or IGES format.

*Original mesh is slightly higher resolution than the file provided here.