I know @SpringerSpaniel had mentioned working on his own bracket for this purpose (see: NVMe U.3 Backplane / solution - #5 by SpringerSpaniel), and he generously provided a download of it… but I just printed it and test-fit it—and the drive orientation being front-to-back means the connectors would not fit on the drives with my tower cooler.
There’s also this vertical bracket by @Venson – HL15 Vertical SSD mount by GerVenson - Thingiverse (but it also only works with narrower drives, and has the same cable issue where I wouldn’t have the clearance with my tower cooler).