This may not be the sort of response you're looking for....but are you so determined to carry all your weight way up high? I carry a tailbox about two thirds that size as a matter of course, and just loading it up with daytrip stuff (tools, camera, food and water, a few odds and ends) makes me plenty top-heavy. Adding a large duffel loaded with long distance and camping gear would be survivable, but not much fun. It's about enjoying the ride, right? The things you've seen people do on bikes in Africa or Asia they're doing because they've got no choice, not because they're trying to enjoy themselves.
Before getting to the point you're describing, I clip on back panniers, then front panniers, and I load all the heavy stuff as low as possible; I may add some gear behind me on the seat as a last resort, usually stuff that is bulky but light.
Personally, I'd eliminate the top box in favor of panniers if I had to, not the reverse.
Hope that's helpful.
Mark
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