I've used
Fusion 360 to make changes to the case before. It is a pretty popular bit of software for doing 3D printing modeling and it is incredibly powerful since it is a professional-grade product. A license to use it is free as long as you promise that you're a hobbyist and won't use it at a company that has more than $100,000 in yearly revenue. Autodesk has their own people who put out tutorials, but since it is used by so many hobbyists, there are a lot of third party tutorials on youtube as well. The only drawback is that they have drastically improved the product over the past 3 or 4 years so a lot of the functions in the UI have been reorganized or renamed.
I'd take a stab at making the case you want but I don't have it installed on my computer currently.