Does anybody have PCIe (3.0) aka PCI Express module card edge connector technical drawing?
The official PCIe spec from PCI-SIG has this information. It’s downloadable from the PCI-SIG site for a fee, it’s free download for PCI-SIG members. Link: https://pcisig.com/specifications

By the way, Supermicro lists that kind of adapter (half-high to 2x NVMe.) Here it is on Amazon: https://www.amazon.com/Supermicro-AOC-SLG3-2M2-PCIe-Add-Card/dp/B071S3ZY8P

Check also ASUS, AIC and Funtin. These are ‘passive’ adapters that rely on the host to bifurcate the PCIe lanes - which means newer motherboards only with appropriate BIOS support (intel x299; AMD x399, Epyc, etc).

Amfeltec has an active adapter that can hold up to 4x M.2 in a half-high slot. It includes a switch and it’s rather expensive. It would literally be cheaper to get a different motherboard and use lane bifurcation.

If you can live with a full-height card there are more choices. The ASUS Hyper16 card, which takes 4x NVMe seems to be the most cost-effective. ASRock also has one.