There isn't any space on the end of the bar to clamp the mirror to, nor can you install the internal mount adapter they sell:

because the stock TBS Bars have a (5mm) threaded insert welded in the end. What Mick is suggesting is to fasten a stack of washers to the bar end insert that will create a surface for the mirror to clamp onto outside of the grips instead of trimming or moving the gips and controls inboard to create the clearance.