If file system level isolation is enough for you, take a loot at schroot (https://linux.die.net/man/1/schroot) which allows root-less chroot. You can use something like debootstrap to get a complete userland into a user controlled directory and use schroot to chroot into it without root level access.
> it seems that for creating a chroot you still require root.
You actually don't as long as you have user namespaces.
One thing I am working on I use chroot (rather unshare --root=) to minimally sandbox a subprocess. At the beginning of the script I have this little snippet:
if [ "$(id --user)" -ne 0 ]; then
exec unshare --map-root-user --mount -- "$0" "$@"
fi
Though you can probably just do something roughtly as `unshare --map-root-user --root=<PATH>`.
