I believe that Duct Seal would work for this also. It's available at any Home Depot or Lowe's.

Gardner Bender 1 lb. Plug Duct Seal Compound DS-110 - The Home Depot
Gardner Bender Duct Seal Compound Plugs is a dough-like material that is easy to handle. This material helps seal and protect gaps, holes and conduit openings against dust, moisture, drafts and noise.
www.homedepot.com