I use 'Great Stuff' expandable foam. The regular kind is an ugly off-white/yellow color but they have a black pond and stone foam. It is three times as much $ as the regular one and I have never been able to find any except online. The regular would probably be best since you will probably want to put silicone over it and while the silicone is still wet, press some coco fiber or moss on it to cover it and make it look more natural. I hope this was helpful