Marcos,
I made a bit of an overstatement in my original post and have since corrected it to reflect your reply - thank you.
The modification which I made to the old Nardi was relatively simple and did not require any drilling/tapping. It was originally configured exactly as the one in this original thread; a plunger, but no ring or tapped holes. My car and steering wheel are tucked away in the shop so I can't provide the actual photos but here's a near exact set-up which I lifted from a website:
The brass ring and plastic insulator shown here appears to be identical to the one I used and which I removed from an old MOMO adapter. The wire to the horn button is soldered to the backside of the brass and was positioned exactly the same distance from the hub center as that of the original (early) plunger. I removed the original plunger and it's insulator from the wheel, placed the horn button wire through the (now) empty plunger hole and then pressed the brass ring and its plastic case onto the hub. It's a very snug fit on the boss and sandwiched between the wheel and hub and has never moved an iota (I've had the wheel off a couple of times). I would have been a bit more difficult without the boss (as in the subject wheel) but I think still do-able. The turn signal cancellation tab is another issue and though my memory is foggy, I think there was no change necessary.