It can be challenging when making puzzle boxes as there always seems to be more that you can add if there is still a lot of space inside the box.
Here are two simple methods that can be used in a variety of ways.
Here, on the left end of the Push Button Combination box I added a Brass Rod that easily slips one way or another, well, unless the Brass Rod is not aligned with the notch in the dial assembly poking through the back. I could have weighted this dial and dropped the knob on the outside, but that might have been too much that is hidden with no clues. By adding the Knob on the outside it definitely makes it easier to see that there is a step here. The role of the Brass Rod here is to block the Drawer from opening on the Left Side.
At the right end, I used a simple Gravity Pin, but made to suit this particular situation of blocking the drawer from coming out. There is a small Pin and block that allows the Gravity Pin to rotate. By turning the box upside down and tapping, it will release. Getting the just right setting takes a bit on adjustment during final assembly. Pushing the magnet deeper into its setting can help make it not too strong. I used 5/32" here, probably would have been fine with an 1/8" magnet.
Knowing what direction to go and what directions to not go is also a well served lesson.
Enjoy the shop time!