Although SBCs on the market today are NOT “old” (contra @_pitch) and could be the latest tech, they are resource minimalistic. They are meant for IOT, embedded, robotics, lite computing. That is why they are inexpensive. They can be assembled into clusters, however. Some CPUs are just multiples of Intel Atom cpus. A CPU can be composed of smaller CPUs (cores) with multiple threads (~virtual cores/cpus). I wonder if a cluster of SBCs could be integrated to approach the computing power required for Qubes. There are only a handful (some mentioned in my earlier post) of SBCs with a single socket physical CPU that has enough power. Intel Celeron probably almost could do it as well as AMD Ryzen. Any CPU smaller than that and Qubes isn’t going to work. Xen needs VT. Making VT out of a cluster of CPUs requires very advanced compsci knowhow.
One of the security advantages of small like an Atom is that it doesn’t have ME (@Emily) in it. No need for ME cleaner or Coreboot configurator. It would be a cool project to see if someone could cluster Atoms together, make a generic “homebrew” VT (like Coreboot: “homebrew” as in not corporate proprietary but still from Federal labs), and then see if Xen could run on top of it.
My recommendation is to try an AMD Ryzen (Udoo), which wont have ME in it but is the same class of processor as the i7 Intels that work well with Qubes.*Xen is apparently incompatible with Ryzen.
Some of the other respondents are providing links to mini desktops, basically a laptop without the peripherals attached. Those are nice and portable but why not just get one of the certified laptops?