i have stuck lots of stuff to canvas with gesso (found in all art supply stores). Gesso is like "hodge podge". but way better. it can be purchased with different dried finishes (matte, gloss, semi-gloss) the matte finish takes acrylic, oil, gouge and even house paint very well.
mix the sand in the gesso and apply to any surface that is clean and free of oil. gesso can be built up either by successive layers or in a shallow mold. I haven't tried it in a deep mold yet.
the good thing about gesso as opposed to white glue is the perminance and resistance to moisture penetration and uplift that white glue is subject to.