My saddle height is 790 mm (centre-BB to top of seat), which is on the high side for a 17" frame! An unsupported post at anything near to this height would be dangerous, as the post could easily fail. The lightest method of providing the necessary strength is to make a seat tube extender and brace it with a small strut. The lower mount was fabricated from steel plate. The strut tube was scrap from an old clothes drying rack. The extension tube is Reynolds 531 steel cut from the seat tube of a scrap road-bike frame. Metal joining is by simple silver-soldering with a plumber's gas torch.