Did any of it work for Marc?
The schoeps extension tube has a buffer circuit, ala KC5 actives. The AKG extension does not (based on pics).
I'd say that either means the AKG cap does not need the buffer circuit (because the FET can drive that cable) or it means the akg tube is not sonically on-par with the normal cap+body. I think you'll need a buffer circuit to preserve fidelity.
Regardless, the circuit is the easy part. Everyone needs to repeat that a thousand times.. or re-read the previous thread
Packaging all the guts into the machined collette and having it be servicable so the cable can be replaced is the hard part. There will need to be at least one manufactured insert for the collette guts and for cable strain relief and anchoring. I'd expect one base plate for cable retention, a circuit assembly and then another interface component for the capsule.
Schoeps mounts the smd circuit on a small board: