I'd use Water Plug Cement. Mix it in small amounts at a time and cram it into the space. Use heavy rubber gloves as this stuff is loaded with lime and will burn your hands. It sets very quickly, so don't mix a whole can at once. It doesn't have to go clear to the bottom of the space. Follow the directions on the can for mixing and applying. This stuff is also great to plug leaks in concrete walls even when water is leaking.