We are aware of no perfect workaround for the ventilation problem with the sunken can method of cold storage. Holes should be added for drainage, but holes big enough to ventilate will also provide potential access for insects and/or small rodents. Gravel or hay can be added to the bottom of the can to improve drainage, but the ventilation remains an issue. The lack of ventilation is not a total dealbreaker, but it will limit the types of vegetables that can be stored there, and for how long.
The extension services at University of Connecticut and University of Michigan service offer some information that may be helpful:
We are aware of no perfect workaround for the ventilation problem with the sunken can method of cold storage. Holes should be added for drainage, but holes big enough to ventilate will also provide potential access for insects and/or small rodents. Gravel or hay can be added to the bottom of the can to improve drainage, but the ventilation remains an issue. The lack of ventilation is not a total dealbreaker, but it will limit the types of vegetables that can be stored there, and for how long.
The extension services at University of Connecticut and University of Michigan service offer some information that may be helpful:
https://publications.extension.uconn.edu/2017/11/30/cold-storage-a-sustainable-way-to-preserve-the-harvest/
https://www.canr.msu.edu/hrt/uploads/535/78622/Root-Cellar-Handout-Diagram-6pgs.pdf
You might also find inspiration in https://www.almanac.com/storing-your-harvest-without-root-cellar or https://www.almanac.com/how-build-root-cellar.