Well that was the whole point of the build, was to allow a passenger on without removing the rear seat. I am not the least bit concerned where the weight is. There really is not enough weight total to be an issue. My Harley tourpak alone weighs over 50lbs, and it sits well behind the axle. Even then they recommend a 30lb limit inside the tourpak.
About 90% of ADV bikes have the weight well behind the axle.
Its not the location, its the support. Mine is not as strong as I had hoped. Plus the design of the rear seat pillion allows it to flex as well. The pillion frame is not as strong as I first believed, but neither is the tubing I used. Stronger tubing will only make the pillion frame flex more.
I assume you did notice its not just bolted to the sides of the pillion frame, but to the rear also? There are 3 mounting points. I have also wondered about a brace coming from the side frame of the fenders, but am starting to think, they are probably not very strong either. At least not designed for a load from that direction.
If moving it forward does not reduce the flex enough, I will be forced to go the route of seat removal, like you mentioned. If I have to go that route, I think I can make it a one bolt removal setup for both the box and the seat.
But as I mentioned earlier, I have 2 other projects requiring my attention before I get back to this. One is building a frame and covering our garden with chicken wire to keep squirrels and birds from eating our veggies before we do. The other is replacing the over the air antenna on our RV that tree branches ripped off on our last trip out. I then have to fabricate a guard to protect the new one, all before our upcoming rv trip at the end of the month.
Will update the post when I get started again on this. My panniers are supposed to arrive sometime between Mar 26 and Apr 15, so I will have thos racks to build as well.