I take it your monitor is in a horizontal frame ? Cut your shelf a bit wider than your monitor width , { front to back } and make it the same inside width as your cabinet. Position your shelf inside cabinet where you want it , straight , tilted down in back ect. mark the bottom edge on cab. walls . Get some hardwood pieces , drill and fasten along lines. Put a piece on back of shelf to use as monitor stop. Mark holes in monitor frame, drill and fasten. Screw or bolt shelf to rails, { before monitor} I hope this helps I'm not familiar with your particular cabinet.