I would be concerned with the 1 x 1 aluminium, especially hand-bent. Motorcycles have a lot of vibrations, and bending that thin section of aluminium without annealing (to re-crystalizing the grain structure) isn't a good idea as well.
Whenever you see the mounting hardware for a top box, you see that the mounting racks usually tubular - and usually steel. SWMotech makes a small aluminium rack that fits top cases with adapters, but I've actually heard cases of it fatiguing and cracking with more than 10 lbs attached to it.
If you're interested in this concept and want to do it fairly cheaper, I'd find a welder and bend/weld/fasten some tubes to do the job. Or, get a trunk mount kit for your bike but just top it with a sheet metal plate.
That said, I like the concept and support the can-do effort. Just check it for cracks every so often!