I've been using FreeBSD jails for a while, and was looking for a simple way to create jail filesystems.   After some reading, I came up with this simple script which creates full FreeBSD filesystems.   The filesystem can then be used via jail.conf.

#!/bin/bash
JAILNAME=$1
echo "kjail "$1
JAIL_HOME=/tank/kjail/
FTP=http://ftp.freebsd.org/pub/FreeBSD/releases/amd64/11.1-RELEASE/
# 
# get packages
#
if [ ! -f $JAIL_HOME/packages/doc.txz ]; then
        wget $FTP/doc.txz -O $JAIL_HOME/packages/doc.txz
        wget $FTP/base.txz -O $JAIL_HOME/packages/base.txz
        wget $FTP/kernel.txz -O $JAIL_HOME/packages/kernel.txz
        wget $FTP/lib32.txz -O $JAIL_HOME/packages/lib32.txz
fi
#
# make jail
#
mkdir -p $JAIL_HOME/$JAILNAME
tar zxvf $JAIL_HOME/packages/doc.txz -C $JAIL_HOME/$JAILNAME
tar zxvf $JAIL_HOME/packages/base.txz -C $JAIL_HOME/$JAILNAME
tar zxvf $JAIL_HOME/packages/kernel.txz -C $JAIL_HOME/$JAILNAME
tar zxvf $JAIL_HOME/packages/lib32.txz -C $JAIL_HOME/$JAILNAME