BTA likes to attach and hide its foot under a rock, or in a crevice. Unless you want to isolate it to the shell, I would suggest moving the shell next to the rock you want it to settle. If you try to isolate it to the shell, there is a real risk of it detaches and float away. When this happen, it will probably ends up in the powerhead.
If you do decide to move the shell to a rock, choose a location with low to moderate indirect flow. You should also avoid putting it directly in the center of the light source.
Good luck :)