Stackable Bottle Holder / Collector

Please take also a look to my new design: https://www.thingiverse.com/thing:2618022

This is a stackable bottle holder for one-way returnable bottles, used in Germany, to carry them back to the supermarket. With the displayed 4 levels, you can carry 36 bottles, but there are also more levels possible.
I don't know, how this is handled in other countries, but in Germany the one-way bottles have € 0.25 deposit. So you have to collect them, and bring them back to the supermarket. I made already