Hi robhar and thanks!
You can definitely stack them. Rotopax has a storage container version now too.
I saw those liquid bladders recently here and they look like a neat option.
The stock Rotopax DLX L bracket is being used. There is also a lighter gauge version that comes with the Rotopax 1gal. when you order them together. It is 12 gauge instead of the 8 gauge of the DLX L bracket.
There is some lateral flex in the system. At this point there is no extra support. It is all being tested by the "MOD Squad" now.
Crash damage?...don't know yet. Although the whole setup doesn't stick out as far the passenger pegs so that will offer some protection in a tip over and the flex in the system should give it some resilience.
The L bracket does have some slotted holes. In the pic you show above the L bracket is actually held on by five bolts. There is even an option with the slotted holes to have the Rotopax "angle in" at the tail making it even a little lower profile.
In the pic below it is held on by two bolts and then sits against the BMW Aluminum box rack.
More pics of the setup above here;
I would actually like to see which other side racks this configuration will work with. Between the slots in the L bracket and the five holes in our Top Rack there are a few options.