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.
Amazon.com: Solid State Drive L18845-002 Compatible Replacement Spare Part for Western Digital PC SN520 SDAPNUW-256G 256GB M.2 2280 B+M NVME PCIE Gen3 x2 SSD : Electronics