Solved Installing a custom kernel fails on ZFS

FreeBSD 11.1; Two identical HDD on ZFS/stripping mode, MBR (BIOS).

Building a custom kernel went fine. Installing the kernel fails with the error that it doesn't find /boot and cannot create /boot/kernel.

On ZFS, /boot links to /bootpool/boot. The boot directory did not exist, so I created it. Then the kernel installed but rebooting didn't switch to the new kernel.

What should I do?
Why is this not mentioned in either the handbook or /usr/src/UPDATING? It seems quite an important point!
I'm guessing this is an encrypted ZFS install? The 'regular' ZFS install doesn't have this issue.