Use a USB3.0 cable, which includes an appropriate shielded pair for both TX and RX. DP/DM can be used for REFCLK. It’s not perfect but it will work. This is exactly what a ‘GPU Miner’ slot adapter uses. Example: https://www.parallelminer.com/product/usb-3-0-pcie-riser-card-expansion

If you want your cable to be exactly PCIe compliant, consider SFF-8639 (U.2), iPass, Firefly - all designed to carry PCIe.

USB Type C with the appropriate signal set can also be used for PCIe. This is what Thunderbolt 3 uses.