This is my personal initramfs.
It uses busybox to execute the init shell script, and cryptsetup to
decrypt encrypted partitions.
This script first decrypts the luks.key file (not added to git), which is a
luks container with a very small ext4 partition in it.
This container asks for a password, and provides a key file.
Then, cryptsetup decrypts three real partitions and mounts the root one.
Finally, it cleans the environment and switch to the real root partition.
This method asks only one time for a password, and decrypts several partitions on several disks.
just type make install and it will generate and install the initram image in /boot
You are free to copy it and to adapt it to your needs.