Since the key in /etc/ha.d/authkeys file never has to be typed by a human being, it is not necessary for it to be in any way mnemonic or memorable. As a result a long, randomly generated key is a good choice.
The following line of shell script will generate such a key:
cat <<-!AUTH >/etc/ha.d/authkeys # Automatically generated authkeys file auth 1 1 sha1 `dd if=/dev/urandom count=4 2>/dev/null | md5sum | cut -c1-32` !AUTH
Or you can download a complete command which will do this quite nicely.
Or for SHA1:
dd if=/dev/urandom count=4 2>/dev/null | openssl dgst -sha1