Part that connects two "Kalfas" boxes (in Czech also "maltovnik") from Baumax to create a container https://www.baumax.cz/stavebni-naradi-a-prislusenstvi/?q=kalfas - for example this one
https://www.baumax.cz/vrobek/kalfas-45,57073.html

In Poland it's probably this product https://www.merkurymarket.pl/produkt/kastra-prostokatna-45,44562.html

Need 4 items to make secure connection on each side.

You need only STL file to print this thing.