It’s an old(ish) post (mostly showing how to build the enclosure), and the download link pointing at Bell Labs is dead. Plan9 has a new home at https://9p.io/plan9/.
If you stumble across a labs URL (like in the article), you can usually just replace "plan9.bell-labs.com" with "9p.io". For instance, the link to Miller's RPi build that was linked to should be updated to http://9p.io/sources/contrib/miller/9pi.img.gz
