I accidentally bought an intel NUC (5i5RYK) that only takes an M.2 SSD to use as my mini home server. To squeeze a 2.5" HDD I designed this extension.

You'll need :

SST-CP12 cable - bought one for £6 on amazon
4x M2.5 12mm male/female standoffs (only had 10mm, so added a nut)
4x M2.5 6mm male/female standoffs
4x M3 6mm screws for the drive. Make sure the heads sit in the recesses so as not to touch the SSD/RAM.

Hope this helps :)