The way we use to build them was a piece of clay tile 3’long and 2′ in Dia. and bury it in the ground to grade and put a foot of 1-1/2″ gravel in bottom and 3/4″ gravel on top of that about a ft. and end up about 6″ below the pipe with pea gravel. But look in search and they might have a good deyail because this was in the 50s when we did this.