Power supply for a solenoid to open a secret compartment
I think you may be using the wrong mechanism for the lock. Maglocks (magnetic locks) are widely used for this type of application in building security. They come in two modes: release when power off (suitable for most applications) and require power to release (prisons, etc.).



Figure 1. Maglock with magnetic poles exposed and keeper.

It seems to me that the three poles are arranged as south-north-south (or vice-versa) and that the coil is wound, pushed into the black slots and potted in position. Once the (electro) magnet hits the keeper the magnetic circuit is closed. As anyone who has played with a horseshoe magnet will know, opening the closed loop is very difficult. 



Figure 2. Cross-sectional view.

Here we can see that with the lock open the exposed faces are poles of the electro-magnet. Note also that the magnetic path is twice as wide in the centre pole as on the upper and lower poles so that flux density is fairly constant. Once the lock closes the flux forms a loop through the iron core.


  Some of the suggestions had ways that didn't consume power to hold it in place but still needed it to unlock it.


The "normally locked" version requires power to unlock. The coils are typically 12 or 24 V so that the controller can run with a small lead-acid battery backup. You would need to check the datasheets to learn how much current they consume and purchase a suitable power supply.



Figure 3. A compact cylindrical magnetic lock.

The locking force will probably be rated in newtons (N). You can convert this to kg-force by dividing by 10. (Actually 9.81 m/s², the acceleration due to gravity but 10 is close enough.) Remember that you want the "energise to release" type. These are not the usual types sold.