My goal was to wade back into casual gaming with a capable 1080p gaming computer in a small form factor, and this was easily achieved with this build.
-To make the graphics card fit next to the HDPLEX 400W AC-DC, I had to de-pin the Skywire GPU cable and heat shrink each wire to make ultra low-profile cables. I had never done anything with cables before and it was a little bit daunting, but I used these links to help me and it turned out to be quite easy and fun (I ignored the first heat shrink wrap suggestion in the second link): https://youtu.be/n6gQ5ie2Dw0 https://imgur.com/gallery/dqkanA2
-Cut and fit Velcro to the front panel of the S4 Mini so it would come through the cut-outs in the chassis to allow Velcro mounting of the HDPLEX HiFi DC-ATX.
-The 40mm fan included with the HDPLEX 400W AC-DC would not fit on the back side of the chassis with the included C6 connector, so I Velcro'ed one edge of it to a wall on the front side of the chassis instead with a little space for it to pull air from the front and blow through the AC-DC out the back. I don't think I've ever heard it turn on as this system pulls very little power, and frankly it's probably unnecessary but I liked having it just in case.
-I used extra rubber and foam from a ping pong paddle to cut out a spacer for the C6 connector. I had to get two longer M2.5 screws as well as a nut and washer to mount it correctly. There might have been space for the C6 connector to fit flush but it seemed a little too tight for my liking and preferred having a little extra room with minimal effort.
All the folks on the SFF Forum, the NFC Discord Channel, and especially Josh from NFC Systems were extremely helpful. I hope this guide assists another enthusiast as these people assisted me.