Here's the cargo container I purchased for use as a storage building. The container is 40 feet by 8 feet by 8 feet high. It has heavy locking doors on both ends. The doors also have a shielded box for a padlock. The shielded box will prevent someone getting a pair of bolt cutters on the lock. I have yet to find a padlock which has enough shackle clearance to fit.
The container costs about $3300 (delivery included). You can get used containers for about half price, but they will most likely be dented, have bad paint, and may not have doors on both ends.
Click on images for larger view
I only bought one. The other containers shown are Dad's
The floor is marine grade treated wood. Underneath the wood are steel beams which run crosswise. The steel beams are spaced about a foot apart. You can see the bolts which hold the wood flooring to the beams.
This view should give an idea of the internal size.
The container is rated to support 423,289 pounds of weight stacked on it (at 1.8 times normal gravity). The walls and roof are heavy gauge corrigated steel. The walls also have tie-down loops near the roof and floor.
Here is the nameplates that shows the specs and ratings. The container will carry 58,270 pounds of stuff!
The only downside of the container is that it is almost 9,000 pounds when empty. This makes it quite difficult to move around.