SEEED were nice enough to let me buy a PCIe to M.2 hat for Raspberry Pi 5 for just 0.01 usd and free shipping. It arrived safely this morning.
It is nicely presented in a branded box, and seems to have all the screws and spacer hardware needed to fit it.
The supporting installation documentation at Raspberry Pi 5 uses PCIe hat/dual hat | Seeed Studio Wiki looks comprehensive, but I haven’t fitted it yet.