I think the trick is to go with a generation or two old Supermicro motherboard in whatever ATX case you can scrounge up, and then use either a low power Xeon or a Pentium/Celeron. Something like the X11SAE-F or X12SCA-F (or maybe even older) is plenty, though maybe not quite as low power. I still use an X9SCA+-F with some very old Xeon for a NAS and to run some LXC containers. It idles at maybe 20-30W instead of 5, but I've never had any issues with it, and I'm sure it's paid itself off many times over.
Even better, Supermicro will pick up the phone/answer emails, even if you bought a years-old secondhand server. They have the manuals, and are more than happy to help you out.
