Mahogany from a porch railing is probably something rather soft. Real mahogany cost about 1-200 for a piece about 8' and they are always threatening to put it under the endangered species regs, so some day you may not be able to take your railing across international boundaries, which can be a problem with guitars...

I you are intending on ripping that small a piece in a table saw (easy way to waste 1/8"), be aware that would be extremely dangerous. Short and thick and wide is a classic for a violent kick back. OK if you make enough of them at once that the piece is long enough. Or use hand saw, band saw, or mount the block in a save jig or slit it on your mill.