[AdamHeard]

He has already told us that a clearance issue exists (like in the AM toughboxes). The larger gear of gear cluster A (arbitrary name) is too close to the shaft of the large gear of gear cluster B for any spacer or collar to be added (The small gear of cluster B is on the same shaft as A). Please correct me if I am wrong, but this is what I am imagining.

A roll pin like in the AM gearbox is the least labor intensive solution I can imagine that doesn't have the possibility to move. I have used the press on retaining clips like Travis suggested, but they have the possibility of drifting (You could still use them, just be prepared to verify the alignment often). Snap rings would also work very well, have a lower profile than roll pins, and look better. They do require machining that is a tad more difficult, but still is a pretty simple machining operation.