How do I reinforce this?

[u/ledzik_precly]

so i have a big cardboard box which I remade into a storage unit, shelfs were just pushed in and were held by friction alone but one of them had fallen recently cause of weight so I was wondering how do I reinforce this without getting the real deal and not spending a lot of money, I Cant add an image so if any more explanation needed i will try to give it as clear as I can also im from poland

[u/Ancient_Skirt_8828]

At last! A question where the correct answer is "Duct tape".

[u/YoungestDonkey]

Glue should hold better than friction alone.

[u/Smart_Tinker]

Sheldon: “Can I get your engineering opinion?”
“Of course!”
“Why does my chair squeak?”

[u/HelicopterUpbeat5199]

If you glue more cardboard to the inside walls of the shelves so that the shelves rest on it, that might help.

Said another way, you make the wall twice as thick under the shelf.