After waiting 2 weeks for reproduction shock mounts to arrive, it became clear that the company didn't have any in stock, and there would be more waiting, so I made my own.
The best resource is the eames patent from 1953, "FURNITURE SHOCK MOUNT CONSTRUCTION" (2,649,136).
The patent goes into detail about shock mounts for the plywood chairs, what they're made of, and how they're adhered to the chair, but their construction is similar to the mounts for the fiberglass chairs.
I didn't spend too much time making them perfect copies of the originals, but they are close. If I need to make more of them, I'll use better hardware and make a rubber mold so I can cast 4 mounts at the same time.
Making the Molds:
The inside diameter of a tube of silicone is just about the right diameter, so I cut it at 3/8" intervals. The slices of tubing are tacked in place with hot glue. Then I ran a small bead of silicone and tooled it with a drill bit to form the round-over on the top edge of the mount.
At the hardware store I found some weld nuts that are 1/4" - 20 (coarse thread). A urethane resin will be poured into the mold, casting the nut inside. The resin has 60 shore hardness, similar to the original mounts. The weld nut is stuck down with silicone, and the threads are filled so mold rubber doesn't get inside.
For the mounts I used a two part resin that I had from another project. It's a urethane resin from smooth-on called Smooth-Cast 60D. It's normally light-amber in color, so I pigmented it black (3% max loading). The resin is mixed 1:1 by volume and can be demolded in 20 minutes.
Shock mounts removed and almost ready to install after some trimming.
Adhering Shock Mounts:
I made a quick cardboard template from a side shell chair with mounts installed. Took that to the concrete piece, measured to the edges, got the template centered, and marked the approximate locations of the holes and drew a shock-mount-sized circle around them.
Concrete polished flat at the mounting points, and crosshairs drawn to use as a reference when the epoxy squeezes out and covers up the circles.
Use the base to make sure the mounts will line up:
PC-7 is a strong two-part epoxy, but it failed here, and after waiting 12+ hours for the epoxy to cure, I mounted a base and all the mounts pulled off easily..... The epoxy stuck to the concrete with no problem, but the mounts are completely clean. I think this happened because I shifted the mounts after the epoxy had started to cure, or the acetone I used to clean the mounts caused problems.
The PC-7 is removed with a chisel and mallet, and the concrete is ground down a little bit before trying a different epoxy and making some changes to the mounts. Good news is the adhesive sticks like crazy on the concrete side.
To fix the problem, I drilled some small holes in the shock mounts so the adhesive has a place to grab into and scuffed them all with 80 grit sandpaper. The next epoxy to try is Smooth-On Metalset A4. Mixes 1:1 by volume, cures light grey, and is incredibly fucking strong. Douglas and Sturgess had it on the shelf.
Put epoxy on the mounts and threw them on, lining up the mounts by measuring to the edges of the chair and then making sure they line up with the base. Waited a day to cure and got the base mounted up with no surprises.
Finally mounted for real:
Looking legit for a copy, but it needs reinforcing at the corner between the seat and the back. Flexing it there has created two hairline cracks, so either the chair needs to be thicker and more uniform, or it needs reinforcement in that area, or both. In the next casting I'll experiment with how the concrete is placed in the form, and try to reinforce those weak spots.