What material to use to close a too wide wire tunnel?
Sealing around wires and cables is usually a job for Duct Seal.



I'm not sure of the availability of this outside the United States, but I imagine it or a similar product is available in most places.

Duct seal never hardens, so it can be removed, move, reshaped, and reused. It also will not react with the wire/cable in any way, which often a requirement in building and electrical codes.