I have an 11" x 11" container that is 24" deep; 8" above ground and 16" below ground. The container is solely to protect the plant from foot traffic; it will be surrounded by grass / 6" top soil / deeper dusty soil. The plant itself can be at ground level, although a bit above would be preferable. I can put a large number of holes below grade; was planning on 1.5" diameter, but could do smaller or larger.

So the question is this: vaguely what percentage of space around a plant needs to be non-container for a plant to behave largely like it isn't in a container; or another way of putting this would be, would the plant's roots all grow towards the open places rather than some of them become root-bound on the closed spaces?