This was done in Freecad, the freecad file is also included. Its a replacement watch strap for the Skagen 567LSLB, perhaps other watches use the same straps.
Update 05/20/2018 - Stronger adapter, named adapter-strong.stl, and the correspondending changed freecad-file were added.
The both straps should be printed with flexible PLA-filament, the Adapter with normal PLA-filemant. The Adapter is glued to the short strap with instant glue, i used the slow glue with high ciscosity for this. The metal parts are taken from an other strap from an other watch with 20mm size.
It was done in Freecad using scans of the original strap