Steps to creating a custom project enclosure Answered

I'm in the process of trying to design a prototype for a custom electronics project. I have a 3d printer at my disposal and I would like to print out my initial prototype and at a later time get the enclosures injection molded. I'm looking for some advice on a starting point. Is there some sort of standard model I can start with? or do people typically always design these from scratch. Does any one have recommendations on a place that could do a short run of injection molded enclosures? any advice or starting points would be helpful