As far as how to set up the worldposition behind translucency, here is a super basic example with emissive green, using actor position. Actual implementations will need to pick shadow color somehow or use alphacomposite to decide how to blend, and ActorPosition will likely need to be set via a bone location using an MID, or use 0,0,0 transform local->world or something, depending on how you attach. And you can definitely try to fit the shape better since I just used a cube here:
To support camera going inside the box, use a box with inverted faces, disable depth test in the material settings and use depthfade as a final multiplier on the opacity. This is effectively doing custom sorting with opaque geometry.