Thanks for your responses.
The fanless choices from HDplex, Streamcom, and Turemetal all exceed my 8x8x6" maximum.
The closest case I've found so far is the Akasa Euler S, but the supported motherboard list on their website tops out at 7th gen Intel CPUs. Perhaps the case heatsink position doesn't match the location or height of the CPU in newer motherboards. (I've written to Akasa to learn if the Euler S supports newer boards and CPUs.)
Note to RogueLeader: Thank you, but I didn't see a suitable case using the link you sent.
CORRECTION: I meant to specify "thin mini-ITX", not "mini-ITX". Sorry!
Here are the specs again:
thin mini-ITX motherboard and external PSU
M.2 NVMe SSD (on motherboard)
16GB RAM
Nofan CR-80EH (113m high) or similar height heatsink/quietfan combo -- or fanless case
i5-9600T (integrated graphics) 35W TDP, more okay if the build is inaudible
no graphics card
no internal PSU
no 2.5" drives
no optical drive
no vertical PCIe cards
Ideal dimensions: less than 8" x 8" x 6" ( 203mm x 203mm x 153mm)
The above height maximum is based having enough room for the 113mm Nofan heatsink. But if a shorter heatsink/fan combo is possible that is mostly inaudible at high load (not Prime 95 load), then even better.
