In the Back?
I think in the back is really difficult because you need the cuts for I/O panel, Power, GPU ?
In the front, bottom, or side panel is easy.
You need to make the 3D design of 2 pieces (Top and bottom), then just cut the pieces in a CNC Router.
If you search in youtube are a lot information, for example
(link)
Phanteks have one thats fit in 120mm fan space
(link)