Why didn't you just put screws with heads back in it ?
When you tigthen it, it'll compress the bind nut into the fire wall again.
Just drop some CA on the threads behind the firewall to prevent it from getting loose, but you'll still be able to break it free.

If you have extra nuts, double nut it from behind so it press againt the bind nuts
Or get rid the bind nuts and just use washers.

Put foam against the firewall so the screws won't punture your tank.