Current OVMF blobs are borrowed from the following URL,

http://download.proxmox.com/debian/dists/buster/pve-no-subscription/binary-amd64/pve-edk2-firmware_2.20200531-1_all.deb

OVMF for non-OpenCore boot loaders: https://github.com/kholia/edk2/tree/macOS-NG-v3.

Building OVMF
-------------

# Add tools and build dependencies:
apt-get update
apt-get install build-essential git lintian debhelper iasl nasm \
  python uuid-dev -y


git clone https://github.com/tianocore/edk2
cd edk2
git submodule update --init
make -C BaseTools
. edksetup.sh BaseTools
build -a X64 -t GCC5 -p OvmfPkg/OvmfPkgX64.dsc -b RELEASE

References
----------

* https://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/

* https://www.nicksherlock.com/2018/04/patch-ovmf-to-support-macos-in-proxmox-5-1/

* https://github.com/thenickdude/edk2 (macos-support branch)