Hi,
I tried to get a couple of dozen sunn-style jack cups when I built my monitors, but could not find them anywhere. Conrad said he was working on something, but did not have any as of a few weeks ago.
The original sunn cups are metal, not plastic, and they are sized to sandwich a 3/4-inch surface with no modification. The 412SR cabinets have a 1/2-inch back, and sunn used a 1/4-inch plywood spacer to take up the slack.
Fender has chrome cups, but I believe they are not quite as deep as the sunn parts.
My techie had some single-piece cups, approximately 2 inches in diameter, so I used those for my monitors, which are not original anyway. These cups attach to the face of the surface with three pan head screws, and the sunn hole can be beveled with a rasp or knife so the cup recess will fit inside.
Here's what I used:
Warmest...