Inspired by a modder who managed to squeeze a full tower heatsink into the SG13, I made the same attempt with the goal of being able to have the 9900KF run Prime95 small FFT with AVX at 4.7GHz, without having to drill/cut the case.
The HDPlex DC-ATX unit was attached to the floor in front of the front fan via velcro, while the AC-DC unit rested on the GPU (padded with velcro) and the upper bar of the case frame. Since inserting the screw through the hole of the AC-DC unit's ear made it difficult to close the case cover snugly, it is held in place by the case cover and the screw next to the ear. I replaced the stock 40mm fan with a Noctua variant. I chose a 120x120x15mm instead of 120x120x25mm front fan to make it easier to unplug cables from the DC-ATX unit.
The stock TY-127BP fan that came with the Thermalright Silver Arrow 130 could only handle up 160W before the CPU hit 100C, so I replaced the fan with the Noctua NF-F12 iPPC. This fan is quiet below 1000rpm and is able to ramp up to 3000rpm if needed.
My initial tests showed that due to lack of ventilation in the SG13, having the front fan as intake and the CPU fan blowing towards the back of the case mean that temperatures rose quickly due to recycling of hot air. Therefore, I switched the directions of those 2 fans and added two 92mm intake fans to the top vent.
Optane memory is used to hold the paging file. It's not likely that I will need it since there's 64GB of RAM, but since there are two M.2 slots, so why not.
As for the results, the CPU reaches a max of 97C in Prime95 after 20 minutes, and it is really only one outlier core that is hotter than the rest. The PSU seems sufficient for daily use for this setup, given that I won't be heavily stressing both CPU and GPU at the same time. Also, the HDPlex combo has been shown to be able to power an 8700K and 1080Ti. Interestingly, a combination of Prime95 without AVX and Furmark caused the PC to reboot even though Kill-a-Watt showed the AC-DC intake was 370W, so it was probably a power spike caused by Furmark. On the other hand, I had no issues running Unigine Superposition and Prime95 simultaneously, though I wouldn't recommend it (Kill-a-Watt was showing a peak intake of 470W, which is almost 400W output at 85% efficiency).
My next step is to get a ventilated ATX PSU backplate that also has a hole for C14 mounting for the AC-DC unit, but it seems something as basic as this is really difficult to find.
Update 1: I swapped the Noctua 120x120x15mm fan for the 120x120x25mm variant and CPU temperature dropped to 93C during Prime95 small FFT with AVX. Interestingly, when the setup was tested in an open air testbench, the max temperature was 83C, so it looks like more ventilation will definitely help.
Update 2: I added a ventilated ATX backplate and the Prime95 CPU temperature dropped to 91C.