I may had tower speakers in the future but right now I am going to be adding a bimini cargo top. The cargo unit is from Monster and has to be drilled. I want to leave enough room below the top of the tower for speakers if and when I had them in the future. How much space from the base of the top tower bar should I leave for speakers. 12", is too little. I know it will change with the speaker chosen but no one seems to include this dimension. Give me your measurements, please.