Mounting encrypted drive on boot

My setup consist of OpenBSD 6.7 with full drive encryption using
softraid, configured as described in FAQ:

/dev/sd0a - encrypted volume
/dev/sd1 - decrypted

I have additional need to mount an encrypted /var volume on boot.
This volume is separate drive attached to be VPS "machine".

I want to mount this drive automatically on boot by adding
relevant entries to /etc/fstab, but before this can be done,
softraid device must be configured using bioctl.

On Linux this is done by adding some entries to /etc/crypttab
and the boot script performs required configuration before disks
in fstab are mounted.

How to do similar thing in OpenBSD?

Somebody on StackOverflow advised on modifying /etc/rc
and run bioctl before disks are mounted, but I'm not sure
if this is a right approach, especially that attaching
more disks might change the /dev/sd* numberign.

What would be the best approach?

Best regards,
Chris