The vent holes are on top of the existing breather, which is around 16 1/2 mm dia.
So a bit 5/8" hose fits over it nicely and can be run up to the seat rail. Job done.
As has been previously pointed out the fumes from the breather condense inside the hose and eventually run back down to the vent so it requires regular removal and cleaning.
Not much requirement for it here in the driest state in the driest continent but it looks cool----.
Adelaide Hills, Australia. 93 R100 GS, 77 R75/7 ,70 BSA B44VS, , 86 R80 G/S PD, 95 BMW Funduro F650 ST