File Holder for Ikea Alex Organizing System by printingbuddy.
Print the Big File Holder twice, glue.
The Small File Holder, half the size, can be placed on top or on the side for added flexibility.

Im using these files from Libration:
https://www.amazon.de/dp/B084X6MSTM/ref=twister_B09QQJK9NS?_encoding=UTF8&th=1