We had our roof replaced this summer, and the roofers installed a new vent for the bathroom exhaust fan. It's a cathedral ceiling with a PVC roof, rigid foam and batting in the joist cavities. DH and I finally peeled back the insulation to install a new fan. The roof vent does not extend all the way through the roof sheathing--it stops about 1/4" shy of the bottom of the plywood. How do we attach and seal our fan ducting to this vent?

1. Insert an elbow duct into that vent, reach up through the elbow and to coat the seam with mastic on the inside of the join, and hope that all holds together for a long time? Or,
2. Call the roofers back to insert a vent that clears the sheating by enough distance that we can screw new duct sections to it, and seal the join with foil tape? Or,
3. Something else?????