Sorry, can't give you the answer for the fd-problem, but I can provide a fix:

Why not flock "$fn.lock" instead of trying to lock $fn directly? This way, you don't have to open $fn, so you can store the normal way. For you application, it won't make a difference, I guess. Also, it's quite a widespread unix practice when having to lock around "black boxes" that you can't/don't want to change.