I remixed the model from user Whimsii3D to create a bottom cover. Now nothing can fall out anymore and it looks prettier.
You can now put in tissues without using the cardboard box in case you do not have the right size of box available and the rounded edged match the top part of the box.
The author remixed this model.

Top part of the original model was used to create a corresponding bottom cover. I simply cut the top, filled the elngoated hole and inserted a new one for better handling. Then I added an inner wall to make it hold in the original box (sorrx, can't decriobe it better).