Glenn Herbert wrote:"Complete lack of clay" - where are you in Quebec that there is no clay in the area? I would think there is someplace within a reasonable drive at least that you could harvest several buckets of clay. If you talk to a highway department or a building contractor, they will know where there is clay.
You can use mostly rocks for the mass, as long as you have enough clay or similar to fill the gaps between them and the duct.
I'll make experiments and post results.