Unlocking several LUKS volume groups at boot with one passphrase

My latest Slackware install was installed to several LUKS volume groups. Everything unlocks and mounts properly at boot, but the issue is that despite using the same passphrase I must enter that passphrase for each volume group to be unlocked. Ideally, I'd like to enter the passphrase once and have all of these devices unlock without more prompts.

Excuse me if I overlooked prior posts on the topic, but the only advice I'm finding on LQ and elsewhere recommends /etc/crypttab, which I understand to be a feature of systemd distros only. So what is the Slackware Way to unlock multiple devices at boot using a single passphrase? I'm currently using elilo and would prefer to avoid grub if it can be avoided.