Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:6081911.
Made and tested a Kleenex version (see .stl) and decided to share the parametric .f3d file.
Parameters are pretty straight forward.
If you want a long box, you will need to swap out the mouth by editing the Hole Patterns sketch and then editing the Extrude Hole operation. You can change the number of cubes on the long side by altering the lHorizSquares function.
Lastly, it's a long print and it's intended to be a snug fit. As a test to ensure your box of tissue actually fits the way you want it too and not wasting time and materials, I recommend rewinding to just after the “Extrude Hole” function, generate the STL and then use the Prusaslicer “cut” function to print a small cross section. 6mm seems to be rigid enough to gauge fitment.
Enjoy!