Didn’t do it but to a minimum you will need a dedicated interface for HA check and either a remote controlled power strip or similar functionality on motherboard.
You can have a simple script that keeps the interfaces on the stand by unit down until a failure is detected on the active unit.
Once a failure is detected on active unit, reboot active unit (power recycle) and bring up interfaces on standby unit. Stand by unit become active unit.
On reboot the unit should bring up only the HA interface and check if there is already an active unit. If there is it should remain in stand by, if not it should become active by bringing up the other interfaces.