I did this for *me*. Dell OptiPlex 9020 SFF with Libreboot.
i7-4790k (does not throttle!), 32GB RAM. AMD Radeon RX 6400 (Yeston!). NVMe SSD.
***Heavily*** modified. NH-L9x65 cooler. Fans mounted in rear. HDD/ODD cage removed. Rectangle cut out on front; Inner chassis cut so SATA SSD now screws in at the bottom-front (not the HDD cage that I removed), and intake fan at top-front.
I *will* be selling these. I'm doing more testing; on TODO is 3D print a custom side panel with extra ventilation.
@libreleah What dGPU is it gonna have (if any)?