Hello - I use SolidWorks so can model the housing and provide drawings in whichever format is most convenient for you (PDF, DXF, or DWG are the most common). I would also supply 3D models in an appropriate format so you can see how the housing looks from different angles.
Are you planning to have the housing injection moulded? SolidWorks has a number of features specifically designed for moulded parts, which saves time and helps ensure the finished part is fit for purpose. [EDITED TO ADD: I see from the 'skills' part of the brief you are 3D-printing the design. I have designed a variety of parts that have been successfully 3D printed, either through FDM, SLS, and SLA processes. I would supply .stl files for you to send to the manufacturer].
I would need you to provide full details of the 'innards' - dimensions, positions, etc - to ensure I can design an appropriate and attractive housing.
Please get in touch if you need any further information. I'm keen to build up my feedback profile on Freelancer, so I hope I can be of assistance to you with this project.
Best regards,
Dan.