You can now buy your own Apple Watch strap adapters which will allow you to fit some after market watch straps to your Apple Watch.
The adapters are available in Silver and Black and will allow your Apple Watch to wear Under the Cuff Two Piece* Straps.
Each pair of Adapters will include a pair of silver springbars for you to use with other straps you may own.
The Adapters fit all Apple Watches in the following sizes: 38mm, 40mm, 41mm, 42mm, 44mm and 45mm and will allow your Apple Watch to wear either a 20mm or 22mm wide strap.
The price is reduced if bought with a ParaTac strap via the ParaTac product page.
*Unfortunately, due to their sheer thickness, the Under The Cuff "Original Two Piece Nato" straps are not compatible with these adapters.
(N.B: Whilst it is technically possible to wear a pass-through Nato strap with an Apple Watch, it is not advised, as the strap will obscure the sensors on the underside that need to make contact with the skin on your wrist to function properly)