what i did to sink mine is put one of those thick vegetable elastics around them and attatch somesort of weight to them. i just got a large flat fishing weight and it worked fine. then in my other tank i attatched them directly to a piece of anchored driftwood.
im thinking a good way of sinking it would be to place a thin sheet of lead under it,,, then tie it to the Pipe with clear fishing line... Just shove some gravel/Sand over lead so it cant be seen...!
I still think Bogwood is best option for caves