Good way to dampen tonearm?

I am in the process of replacing the stock Klotz wiring harness in my Rega RB-900 with the single-piece "Incognito" wiring harness. I have been thinking about adding some damping material inside the tonearm tube, and considered trying a couple of shots of expanding insulation foam.

I'd appreciate comments about the wisdom (or lack thereof) of the foam treatment idea. I am concerned that the chemicals in the expanding foam might be bad for the dielectric on the very thin Litz wire inside the tonearm tube.

If anyone has any ideas, or personal experience, with ways to dampen arm tubes, I'd appreciate getting your commentary.