What about if you split the middle and outer parts into separate parts, and put an attachment point in the center of both of them? Then you could put the center on, surface attach any things you wanted on it, then attach the outer cage to that middle attachment point, and be able to surface attach parts to that too. I made a mockup by editing existing parts' attachment points (I had to cheat to get the outer part to place inside the center though): Move the bottom attachment point to the middle. Same with the outer part. Surface attach some RCS tanks and attach the outer part's middle attach point to the center part's. Then surface attach more RCS to the outer part!