I harvested a PCIe Gen3 x2 NVMe 1.3 SSD from my older laptop, the problem is that its length is short. I've held it in place with some tape, it should hold.
Transcend 480GB Jet Drive 820 SSD Only for MacBook Pro (Late 2013 - Early 2015), MacBook Air (Mid 2013 - Early 2015), Mac mini (Late 2014), Mac Pro (Late 2013) - AHCI Gen3 x2 | Megamac