I've considered building one, but as you know...we Festool users are a tad-bit OCD... I find that nothing works or sounds as well as a very expensive set of in-ear-monitors combined with an iphone (so I can still deal with customer calls and texts). If you pick the right phones you can get 26dB or more of noise isolation when your tools are running too. It works great for me but I've yet to run into another "handy guy" that would rather hear his own music than his noisy tools.
I'm a musician and I've been playing with in-ear monitors on stage for 10 years now. I also use them for mowing the lawn and if I'm going to be doing long operations in the shop where I would otherwise need hearing protection, I often grab them if some tuneage would be helpful and not distracting.