3' x 3' x 3' is a cubic yard. Would take more than 10 SDBs at the bank to equal that volume. Maybe a free non working freezer in your garage until you decide.
I did just that in a 10'X24'. We are still working on the inside finishes though. I am sure there is better, but I opted for 3 Rhino Ironworks Safes inside with humidity control in each along with an ERV system.