A different take…
On my Colnago tandem, the stoker has no water bottle bosses. Plus the twin diagonal smaller tubes like a mixte.
I used two thin top clamps meant for a kick stand, ran a recessed brake bolt nut between them and attached the water bottle cage to all that which holds the whole thing together.
I also lined the clamp with electrical tape to avoid scratches.
I’ll look for a pic.
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"Leave the gun. Take the Colnagos."