For DIY you don’t get much choice other that the Asaka cases, see https://www.avforums.com/threads/intel-8th-gen-i5-nuc-akasa-turing-fanless-case-build.2228766/
Tranquil cases don’t seem to be available anymore.
However HDPlex has added a NUC backplate for its H1 case see https://hdplex.com/hdplex-h1-v3-fanless-computer-case.html
That would look good, and I had to have my Roon Core machine in my ‘frontend’ what I would be looking for.
Alternatively there was a 2nd hand Nucleus available FS: Roon Nucleus
Alternatively there are some prebuilt fanless NUCs
See https://simplynuc.co.uk/7i3dnfe-full/