It's sort of shaped like a porkpie hat. The "brim" is as wide as the OD of the spring, and the "hat" part just fits into the ID of the spring. The part is drilled & tapped axially. About 240 degrees of the "brim" are cut away. The entire "hat" is about .010" thicker than the space between the spring windings, so it needs to be forced in between, so it snaps in satisfyingly. You snap it in and then "thread" it down the spring helix until it jams at the end. At that point you can tighten a machine screw into it. The "hat" needs to be tall enough so it holds 3 full threads, and the "brim" needs to be thin enough so it can be coerced into turning to 120 degrees from the end of the spring wire. The spring retention nut I designed will only sit square to the spring axis if the spring has a square end (i.e. if the helix angle of the spring tightens up at the end). The nut sits inside bearing on the part of the spring wire that is square to the spring axis.
I can't really photograph it and do it justice, my explanation is going to have to suffice.
I'm surely not the first one to do this. And I have to make 12 of 'em. I was hoping to find a commercial product, or something that is easier to fabricate.