You'll find one answer to that in the 650 Garage Carb Guide--links are posted in the Tech section here. To give a bit more detail, one simple way is to make an L-bracket joining a breather box bolt to the butterfly-shaped bridge between your carburetors. For vibration damping, drill the holes in the bracket and bridge large enough to line them with a piece of rubber tubing, and use rubber washers at the top and bottom of the assembly and between bracket and bridge, eliminating all metal-to-metal contact.