Boxes for Keter Connect system.
Unfortunately, there's an extra ridge for the first row in the Keter case so these does not fit there easily. But you can use them for the 2nd and 3rd row.
This was used for this product:
https://www.amazon.de/-/en/Keter-233350-Connect-Mobile-Workbench/dp/B06ZZ4FBRL/ref=sr_1_1
The author marked this model as their own original creation. Imported from Thingiverse.