Since I have a backup forestay I can take some risks: even if the furler will fail nothing serious will happen.
The drum is a standard aluminium v-belt pulley (type Z). This pulley is 8 cm diameter. I would rather have a bit bigger pulley, but couldn't because my lathe is not big enough to handle bigger diameters.
I drilled 8 holes through the pulley (hope the photo shows the idea) to make sure the furling line won't slip. Big succes - the line locks in the pulley beyond my expectations.
Other part I'm not too happy with is the tang on the topside of the furler. there was not enough space above the bearing to just 'bolt through', soI fixed the tang with two short M6 bolts. Because the shape of the tang (wide, v-shaped), the forces on it will be quite high. I will probably think of another solution, but not before I have done some tests to see if the furler works.