[jyl]

You can still use the cleat idea, just on the inward flanges at the ends of the shelf. I’d screw a horizontal wood board (2x4) to the studs, the wood board should be the length of the shelf’s inside dimension, and the vertical ends of the board relieved just enough for those flanges to slip between the end of the board and the siding. The top surface of the board will support the shelf, the flanges will keep the shelf from coming off the board at the ends, and the shelf’s stiffness will keep the shelf from coming off the board in the middle.

Add a screw or two through each end of the board, to pinch the flange against the siding, to prevent any rattling. Or, bend the outward flange along the top of the shelf just a little, to create friction as the shelf is slid down onto the wood board. Clean look, no exposed fasteners, no drilling your nice shelf, can put shelf not centered on studs without having off-center fasteners showing, and shelf is removable.

Oh, if you put washers between the board and siding, you don’t even need to relieve the board ends.

Oh oh, if you skip both the washers and the relief, but only partially tighten the lag bolts holding board to studs, so that there is a gap that you can slide the shelf flanges into, then finish tightening the lag bolts - now the board holds the flange tight. But the shelf isn’t as easily removed.

That’s it. 2x4 cut to shelf dimension, lag bolts through 2x4 at studs, done.