You could get a case like these
https://www.newegg.com/black-supermicro-cse-512l-200b/p/N82E16811152222
https://www.newegg.com/istarusa-d118itx-30/p/N82E16811165293
But it may be difficult to securely mount the hard drives. You might need to 3D print some brackets to align with the motherboard mounting holes. Then if you have an SFF-8088 SAS port on the back of your main box use a cable like this or a variant of it and thread it through a hole in the new case.
https://www.amazon.com/Mini-SAS-SFF-8088-SFF-8482-Power-Cable/dp/B08NGJ7F5K/?th=1
Honestly you are really limited with 1U and not a full length rack. In the long run it would probably be cheaper to replace the rack if your storage is going to continue to grow.