Yes I agree about the brackets. I'll have to look into the T-nuts.
The idea I like is you can add on or make adjustments or what have you. Also they would be square which can potentially be troublesome when welding.
Not that it matters now, but I consider the whole idea of screwed together extruded aluminum to be a very expensive way to do a job poorly. We use this stuff everywhere at my job, and it is constantly falling apart, stripping, sagging, rebuilt, rinse, and repeat. I would never use it for a machine base. We’ve done it here, and it invariably gets rebuilt with welded steel.
For an application such as something like a safety cage without any doors, it works decent. Put a door on it, and you may as well start building the steel version. Put any weight on it, especially weight that moves or vibrates and it’s worn out in 6 months.