`Much ado about not much..
If it were me, I'd dig down around the rockto good base (you mentioned it wasn't deep ) about 14-16 inches deep , or to a 3X3 foot square at least 6 inches deep (reinforce if you feel like it), whichever came first and pour a concrete slab to grade level or slightly above and either put in a - as mentioned before- custom built to tie adjustable device, or block to specs.
That rock seems golden to me.
And that's overbuilding I think. The best way to do things. It is after all a mid- support. The screw up or down devise would be more in favor with me than blocking. Simpler down the road. If ever needed.
