build a custom installation image for ARM RBPI

Hi,

I have a RBPI 3B+ running FreeBSD 13 RELEASE. On top of this I have added some software
and configurations we are developing for FreeBSD to capture data from different industrial devices.

Now, I would like to take and package this installation, like a ready image which I can burn to 
a new MicroSD and boot another system off. And that means it should be small in size, as the original
ARM64 FreeBSD Raspberry PI image. 

I had a look to release.sh under /usr/src/release but this means I should rebuild the kernel and userland
, right? I dont want to do that.

Is there anything simpler, to take for example a 32GB FreeBSD 13 vanilla + sfw installation and make it
like a generic image for RBPI3B+?

Thanks,
Stefan