I know someone in the biz, so two other ways people are making money:

1) Buying land and getting it rezoned for storage and then selling it on -- it's getting harder to get permits to build storage as people get sick of them, so there is a whole economy in knowing how and where to build them and only dealing with the paperwork.

2) Storage unit construction is a huge business now.

Sell the shovels in the gold rush etc.