Giant Propel has an aero seatpost, and neither of three supllied Varia ' adapters' can fit onto it. So I designed and printed my own adapter. : r/bicycling
Dual-Position Aero Seatpost – Redshift Sports
lhunter.com Aero seatpost taillight adapter
Corima Seatpost Aero Ellipse - 27.2mm Size M + new VURX SHIM | eBay
Exposure Aero Seat Post Bracket For Tracer | Light Spares | wiggle.com
Giant Carbon Seat Post Clamp Adaptor - In The Know Cycling