TwoMany, thank you for those links. I knew there had to be some threads on this. However, the rubbers from those Lowe's flexible PVC couplings looked really promising to me, so I continued down that route:
Here's the rubber from this one:
https://www.lowes.com/pd/Fernco-3-in-x-3-38-in-dia-Flexible-PVC-Coupling-Fittings/1000075375
The rubber on the other one is too thick, and will be going back to Lowe's.
I wrapped a strip of 18mm masking tape around it as a cutting guide.
I put the rubber sleeve on the gauge for support, and cut my damper piece from the sleeve. The 18mm damper fills a 16mm gap, so there will be 2mm of "squish." It turned out to not quite work out that way, but I still like making the damper 18mm wide.
So, I gave this very simple damper version a try, but assembly got hung up by the raised inner rib in the dash bracket.
I decided to try splitting the damper in two, with one piece above, and one piece below the raised dash bracket rib. I turned version one of the damper inside out and slid it back on the gauge for support. There are raised rings on the rubber sleeve -- I used one as a guide for splitting my damper.
Here is my finished damper. One piece is 6mm wide, the other is 12mm.
And here is the assembled gauge.
Does it "dampen" exactly like the original? Probably not. But it does look great, and there is zero meter slop. The Lowe's flexible PVC coupling cost $4.86, and one coupling is enough to do both gauges.