Hi Elnathan,
I was planning to do a how-to on the couplers but got tied up with some other projects.
I basically cut the hook off the Rapido coupler, drilled a .040" hole in the center of the face into the shank of the coupler. O then drilled another hole on each side of the first hole. I then trimmed and filed them out to form the pocket for the link. Pieces of plastic were glued on the sides and top, then a hole for the pin was drilled and the coupler was filed to shape.
I wish I had taken photos as I built them, it would help show how they were made. I plan to make more of them in the future, I will get some photos then.