I build garage shelves out of 2 x 8 plywood (rip a sheet in half) The long side is supported by a 2 x 4 screwed to the studs. The short side is also screwed tho the wall studs. Then I run a 4 x 4 post up the outside corner and half-lap both joints. I notch the plywood and screw it all down. At one point I had 2 small block engines sitting on the lower shelf.
Looking at your model, I would buid it around a sheet ripped in thirds, so a 16" deep shelf.
No knee braces needed.
