Answer: All of my research indicates the roots of plumeria are non-invasive so there should not be a problem planting in a confined area. The root mass is rather compact. Remember, though that plumeria is a tree, not a shrub, so you can expect the root mass to be equal to the canopy spread. You can prune plumerias back quite hard to keep them compact and in doing so, the roots will remain compact.
Best wishes with your landscape~
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