Whonix has added Monero to its APT repository (you can see that here) so the process is now pretty straightforward:
Download and add the Whonix signing key:
sudo apt-key --keyring /etc/apt/trusted.gpg.d/whonix.gpg add ~/patrick.asc
Add Whonix's APT repository:
echo "deb https://deb.whonix.org buster main contrib ...