OK, here are my "homebrew" actives for Studio Projects C4. I cut about 1" off the end of the mic and used the threads and capsule pin from the original mics. I used a simple circuit: a single FET (self biased one, scavenged from a Transsound TSB120A capsule), a 1G resistor, and a simple voltage drop (zener + resistor). The connection to the mic is three wires: Bias (approx 40V), Audio out, and ground. It has a Binder 711 series 3-pin connector so it can plug into my ancient Beyerdynamic CV750 bodies. Everything is potted in epoxy now...
Added: What shielding is needed? Not that much, in my case the back of the collette is open. Keep flux off (or clean) around the 1G resistor. Use shielded cable. (I used Audio Technica AT853 wire, with both foil and partial braid shield). Finally, to ensure good contact between mic body and ground, I drilled a small hole on the collette, just below the threads, and put a small screw there.