For the look you want- a "Floating shelf" for all intents and purposes- a torsion box construction would be the best design. The sacrifice you have to make would be on the reveal or thickness of the top's supportive structure. I would go as narrow as possible, so a 24" width is the max, and to save weight I would laminate the countertop material (high pressure laminate) sheet directly to the top surface. A hollow-core interior door is a good example of this construction, lightweight but strong and stable. Just a guess, but I think the reveal would be about 5-6" in width, a prototype sample can easily be fabricated to confirm your dimensions and load-bearing needs before you build the real thing.
You sure you don't want to rethink 45-degree lower brackets?