When I have made the Slurpee lid houses I have used enough silicone that the edge is covered. I have also simply "buttered" the lid with silicone and rolled it in dry aragonite sand. I always use aquarium silicone. You can find large plastic barnacles in LFSs sometimes too which fit over the hole for a nice natural-ish look, and are usually cheap. If you use a barnacle, glue it on first and then glue the sand and rubble on around it.
I have taken a jigsaw to these barnacles making a bunch of Jawfish "doors":
