This is a cable gland of the type where "fingers" are pressed against a gasket by a cap.

The nut attached to the core of the gland is supposed to be used when screwing on the cap while the loose nut is used to attach the gland core to box from the inside.

Most important parameters are given in a spreadsheet in the supplied FreeCAD document. Probably, you will want to reduce the length of the core (which is currently 20 mm, corresponding to a common component size). Obviously, this value should be adjusted upwards for thicker walls.

Update 2018-02-08:

All files have been updated. The cap and nut dimensions have been modified to give higher strength. The previous version had a problem with the top of the cap easily breaking off when printed with PLA.