Some time ago I designed an “as small as possible” adapter to make use of spare OCuLink/U.2 ports that are often seen on PC motherboards these days. There are of course ready made products which more-or-less do the same thing, but they were all oversized, awkward and difficult to incorporate into an SFF build.
The unit above is built by hand. If you are considering attempting such a thing note that you will have to get the stencil made and use solder paste to solder the OCuLink connector and DC-DC converter as both have pads underneath which aren’t accessible with a soldering iron.
If you are considering basing something similar on my design, check out the links on this page. This type of PCB design is not for newbies.
Check out the project here on GitHub.
I also designed a PCIe Bifurcation riser. More about that here.