It doesn't really matter - you can have them need to apply as much focus as you want. Physically and magically, all it needs to be is a focal point of their harbinger power - everything else is up to you.

For instance, Steven's is going to be a giant flower. Some sort of tower, summoned creature, or magical device suits just fine. Its form will depend on the Harbinger's powers, and can really be anything. The Star Guardian will want to either convince the Harbinger to stop, or destroy the beacon.

(It's like a miniboss fight of your own design.)