Building a 10' x 12' Harbor Freight greenhouse. The base is 4" of 3/8" crushed rock. I like this size rock as it packs nicely. The foundation is 4"x 6" pressure treated posts.

Here is a link to the greenhouse: Harbor Freight

And one to a nice blog of someone who has put one together with some good tips, reinforcements, and add ons: Harbor Freight Greenhouse blog