[robespierre]

That material is commonly called Silastic, a trademark of Dow Corning. I believe you can purchase a version of it currently under the "DOWSIL" brand. 738, 739, 748, and 3145 RTV look like they would work, from lowest to highest viscosity respectively.
An alternative would be Henkel Loctite 5510 (clear), a modified polysiloxane elastomer with low conductivity and low dielectric. It's firmer than DOWSIL, but thinner uncured for easier application.