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.
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SKHynix 512GB 256GB PCIe Gen3 x 2 M.2 2242 NVMe SSD HFM512GDHTNG-8310A BA SSD | eBay
SILVERSTONE 6 Ports SATA Gen3 (6Gbps) Non-RAID PCI Express Gen3 x2 card - iPon - hardware and software news, reviews, webshop, forum