Depending on your soil, you could dig a hole 2 feet deep. Build a wall around the edge of it big enough for the greenhouse to sit on. Line the floor with concrete blocks (they are porous and will allow water through in case of flooding). Your soil also has to do the same. Angle iron bars resting on the wall can become staging support. I grow everything in pots in the greenhouse - easier for sterilizing. The wall gives added height - my tomato plants regularly reach 10 feet high and average about 10 trusses. You will still need to bolt your greenhouse to the wall.
Bigal