I have built 2 rooms with a soundproof window between them. I have
buried a 2" diameter PVC conduit under the concrete slab between these
2 rooms, and the opening on either side of the conduit comes up to
grade level in both rooms. I am passing sound system cables from one
room to the other through the conduit.

Now that I have passed the cables through, I like to stuff/plug up the
openings of the conduit with some form of putty material. This material
should remain soft so that I can take it out again when I need to add
or remove the cables.

Any suggestions on what material I can use to plug up the conduit
openings? How about plumbers' putty?

Thanks