Then you'll probably want a clone; which will cost in more space. Just remember to remove the remotes.

You could always use an overlayFS with the main (non-worktree) repo as the lowerdir (and then remove the remotes in the overlayFS), but that relies on you not trying to keep working on the git repo at the same time.

Or if I want the sandbox to be able to create commits, which is extremely useful.

Mount the parent read-only.