Simply put, if you have concerns about the DEPTH of your sub causing problems, put something that can act as a spacer. Bricks, rocks, whatever can fill in the space you don't want your substrate to go. If you follow the above approach with lava rock, use big chunks on bottom, and slowly fill in the gaps between them with smaller and smaller pieces. Break some of the stuff up with a hammer or 2x4 if you are as detail oriented as I am. Then you won't have big holes and pockets, and the surface of lava rock is porous and resists shifting. It's not a bad filler. Just be aware that, as people above mentioned, the sub that you put on top of it all will settle down into the rock if there are gaps for it to. And the surface of your sub or cap will want to settle over time. About the only way I know of for keeping slopes is either with barricades hidden in the sub, kind of like terracing rice patties on a hill, or using a thick matt of carpeting plant to root things in place.