Makes no sense that it needs constant power. After the solenoid moves the rod to the lock or unlock position, the rod should stay in place. The solenoid will not influence the rod one way or the other with current going through it.

I don't understand how or why you intend to use a solenoid to retain a sub box. But if you're set on it, use the solenoid to slide a pin into or out of a hole the box. The pin will be loose when the solenoid is not powered but if you design it correctly, there won't be anything to move the pin other than the solenoid. I dunno... a bungee cord seems much simpler.

Sixto
95 S420
87 300SDL