Before I start, all credit to those who pioneered this conversation at the other site long ago. I'm leaning heavily on their observations. I think I may have a slightly simpler solution though. Of course if you're particular about things being factory precise you may want to do something different.
The key elements are a conical spring (Part #7) and a nut/washer (Part #8) combination of some sort that will fit inside the narrow end of the spring. I'm using a T-nut that I modified slightly for this purpose. Neither is difficult to find. There are links in my previous posts to the parts that I ordered. I live in a small town. In a bigger city you might find stuff like this more readily available.
Starting with the t-nut I ground off the prongs and shortened the overall length to approximately 6mm.
This allows the shaft of the t-nut to fit nicely inside of the narrow end of the spring. No need to crimp the spring onto the nut. Once assembled it's not going anywhere.
The spring/nut is then placed inside of the STAY, seat setting (Part #12). It holds itself in place quite nicely.
Thread the LATCH, seat (Part #6) into the nut. You can see here how it would all look when tightened together. In the 2nd and 3rd photos I'm just holding the latch in place with my hand to demonstrate how it will exit the back side of the stay. If I were to let go the spring would just open up again. I mention this because the photo might mislead you to think that somehow the threads of the screw are doing this or that the t-nut is fixed to the stay...it is not. Hope this doesn't cause confusion.
Now onto the frame. (not attached to the seat so that it's easier to see the latch without the seat in the way) This combination is rock solid on the frame. The Stay is sandwiched firmly between the Nut and the frame with no movement.
A couple of notes.
-On the other forum there was speculation that the parts fiche may have the orientation of the spring/nut incorrect. I tried it both ways and I'm convinced the parts fiche shows the correct orientation.
-On the other forum there was conversation about the Clauss reproduction not allowing the nut to spin freely on the small recess at the end of the thread. I've never seen an original LATCH but I can't see how allowing the nut to be loose would be an enhancement. It was speculated that would keep things from inadvertently becoming separated from one other but I can't see that happening with my set-up. It all stays together quite nicely and I don't think it would ever come apart.
If you don't have to pay for shipping the parts should cost about $5.00 and the total time investment would be under 30 minutes...if you took your time.