Case closed.

Cut down and sanded the hardware standoffs to make room for the micro-atx motherboard that just barely fit when mounted to the top of the case.

I used these zip tie anchors to create the mounting points for the motherboard.

Test fit the motherboard. Again, this is a micro-atx motherboard mounted to the top of the XBOX case (mobo is upside down when case comes together).

With the motherboard mounted to the top of the case, the PSU, GPU, and PCI extension are mounted in the original RFI tray from the XBOX (after I cut out slots for fans and I/O and painted the tray flat black).

Getting to the trickier parts of the build; all cables are routed from the lower case over to the upper case. Plenty of space on the outboard sides of the GPU and PSU for some 60x60x10mm fans that I found on amazon.jp (not currently installed).

Applied power to check and make sure everything was working together. I eventually had to cut the sleeve off of the 24 pin connector to get the wires to lay flat enough so they wouldn't interfere with the memory.

Looking at the other XBOX PC builds, I didn't really like the look of the backs with the casing cut out for the motherboards I/O. I found these USB & HDMI extension hubs on amazon.jp; they're used for Subaru vehicle's entertainment systems. I drilled 60mm holes where the power and network connectors went and they mounted in great. It's a minimalist approach, but I liked the way it turned out.

I wired the original controller ports to connect to the USB 2.0 headers of the motherboard. The original wired XBOX controllers work.

The USB/HDMI extension hub not installed. The other side has two USB 3.0's. the excess cables are hidden under the motherboard in the upper casing.

An issue that I've read about with these builds is the thermals. In addition to the CPU cooler fan and the two fans on the GPU, I installed five 60mm fans. The fans are pulling outside air over the GPU and PSU and one fan in the back is exhausting it out. This is FFXIV (not a very demanding game, but the one is played the most) left running overnight with the framerate unlocked. With the frame rate locked at 60 fps, the GPU stays around 67-68.

I had to open up the XBOX after using if for about a month because the dongle for the keyboard I was using stopped working (FYI, I plugged all the dongles I'm using into the USB 2.0 motherboard ports inside the case). This gave me a chance to take some pictures of the fan setup I have installed by the PSU and GPU.

All of the fans are 60X60X10mm japanese brand fans I got on amazon.co.jp. The four fans installed on the sides pull in air over the PSU and GPU. The one exhaust fan is pulling hot air away from the CPU and all other hot air is passively exhausted out the front vents where you see the rgb light coming out (see photo 1).

After looking at the condition of all of the cables when I took the case apart, I found out why I was having such a hard time getting the left side of the case to close. One of the USB 3.0 cables I have plugged into the motherboard was hitting the top of the 60mm fans on that side. To remedy this, I cut off the plastic covering to the cable end that was pressing against the fan.

Just to give you an idea of how close some of these components sit next to each other, I have to fold back and tape the cables for the extension hubs so the PSU would fit.