DIY? I know they have corner shelves, but the one we have isn't anywhere near 17". Get some strips of wood maybe an inch or so deep and 6-8 inches long. Run these level out from the corner. Cut a triangle with 17" sides which should leave you plenty for the speaker as I'm not sure the given depth. Before mounting this paint the strip the wall color and paint the shelf a different one. now mount the cut triangle piece to the strips. Might cost ya $20 at most if you don't have paint around.
SC