You can do this with only a NVMe drive, with some initial trouble. It's the process I use to reconstruct the BootROMs for everyone.
- Dump your BootROM with ROMTool
- Extract the NVRAM volume with UEFITool from your BootROM and the Apple new one.
- Extract the LBSN_BD sector from the old one
- Open the old and new NVRAM volumes with a hex editor, select the Base_XX/ssn/hwc/son block from the old and overwrite the new NVRAM with that. Save the new. You created a base NVRAM volume to use with every new Apple BootROM.
- Open the new BootROM with UEFITool, replace the NVRAM volume with your populated one. Replace the LBSN_BD sector too. Save the reconstructed new BootROM.
- Inject the NVMe DXE to the reconstructed BootROM
- Use ROMTool to flash the new reconstructed BootROM
With the populated NVRAM volume and the LBSN_BD sector, you can reconstruct every past or future LOCKED.fd file from Apple.
I'm gonna detail this process soon, I already wrote most of it, but some things need to be streamlined. I'll will do a video to help too.