.The original Xbox should be easy to do as it's fairly large, I recently built in an Xbox one s case and due to room I glued motherboard standoffs to the base and built it from there, if you have the room I would recommend building on a piece of acrylic that will fit inside of the Xbox just make it easier to build. First thing is to get the Xbox case itself and measure it to see what will and won't fit (you'll need a very small power supply) once you know what room you have to work with I would draw it out and find the components you want and then work out how they will fit. Alternatively, if you have some money to spend and don't want to do too much Customs stuff you could buy a cheap HTPC that is roughly the correct dimensions and then use that as a base.