PeterA: although it's not strictly a headphone question, you might ask this at the headwize.com DIY forum (after all, HeadWize has hosted the article). Jan Meier is a damn smart guy, and he does post there.

And Paul, your glib response makes me wonder if you missed the "design" section of the article, which answers your question directly.

Probably not a design I will personally build, but that's why it's DIY, right?