Hi Martin, i am using carbon tube for boom, the deflection is very low. Is not easy to cut carbon tubes, the best way for me is using dremel tool with CUT-OFF wheel.
I thing that i made the joints little bit oversized. They are made from PET-G filament and are very strong.
I put the STL files here: Fiberglass Ultralight YAGI Antenna by ok1cdj - Thingiverse
73 Ondra OK1CDJ