One of the most difficult parts of any latching box prop is getting proper alignment of the solenoid and the catch that holds the lid shut. After trying many different methods, I've come to this one as the easiest to align and most forgiving. It is the humble cup hook.
Here's a cutaway of the solenoid mounted inside the box with the hook engaging the shaft:
The cup hook's round profile slides over the shaft, compressing the spring slightly and allowing it to slip past and engage the locked position.
Since each box is different, I can't provide precise measurments that will work in all cases. Instead, try moving both hook and solenoid around until you find a position that will work where the lid can open and close freely, and still engage the shaft with enough room for the shaft to throw open.
Here are some photos to serve as a guide.
Mark positions with a pencil and then create a starte hole with an awl or nail.
Test the solenoid position freely or with some poster putty underneath to find the correct placement.
When you find the spot, mark it and create a starte hole, then screw in the solenoid using two of the screws that came with the reed switch. You'll be able to fine tune the hook to dial thing in to perfection later.
You may add flat washers between the box wall and solenoid to adjust the left-right position of the solenoid in case the hook can't be moved any further right and it won't align properly.
Now, try rotating, turning, angling, and shifting the cup hook until the action works smoothly and latches securely. This can take some patience, but is well worth it! Once the position is ideal, you can use hot glue at the base of the hook to prevent it from shifting.
Now that the latch positioning is complete, you'll wire up the switch.