Model a skin (cover) for an stl

Shapr3d newbie, Cad newbie (although I have created 10 + models in OnShape).
I have an stl of a portable radio transceiver that I want to model a skin/cover (think of a cell phone case) that I will eventually print in TPU. I have imported the stl file for the radio but I cant figure out how to wrap the stl. I have tried offset face that does not seem to work for this. Any suggestions of what I can search for to learn how to do this?

Thanks
Lou

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