beam racks for oversized posts

I've just upgraded to a Scott Scale 30 which I'll use for commuting (not as bizarre as it sounds - half my journey is through parks & woodlands and non-one would give a second glance if I upgraded to a nice car to drive in).

Anyone out there know of a beam rack that will fit Scott's oversized (34.9 mm ) seatpost - only needs to be lightweight to carry a few clothes/phone /diary etc. Otherwise, I'm thinking of building one from carbon fibre - bike is v light so stupid to go hanging heavy things on it (other than myself of course!)