Here is how to make a laser cut box, with a better bottom, that can take more weight than other laser cut boxes.
Step 1: Loose Bottom
On the image is shown how I attached the bottom, and how it is normally attached. Normally one uses finger joints to attach a bottom, but the only thing that keeps the bottom in place is the friction in the joints and the glue, but that can easily be broken, so I made a solution where the finger joints are moved up, so that there is wood underneath the joints that takes the force from the bottom, making it more sturdy.
Step 2: Uneven Bottom
The problem again is that the bottom of this box can easily be uneven, because of the wood being thin. So to solve that I made the sides of the box go up in an arc so it rests on four points, one in each corner.
On the top is my solution and on the bottom is the normal one.
Step 3: Assembly
Use the same assembly method as in https://www.instructables.com/id/Laser-Cut-Crate/.
Step 4: Done
Here is the result of my box, now make a box the size you want, using my method.