The used plastic shed is almost not worth the money savings. One place I tried was I put it on an existing concrete slab. Of course, it was not dead flat. Bigger problem that I thought it would be. Been shimming as advised in the instructions. Using my biggest dead blow hammer as my long handled steel sledge is in a trailer in another town one hour round-trip away. Not enough oooph!
So a trip to Harbor Freight 10 minutes away and $8 later I have a 2' wrecking-bar, Great lever. Boosted up the edges and shoved in the shims. One more side to go but it is in the sun so it will have to wait until Monday morning when I can do it in the shade. Can't work in full sun. I get light headed and can't see. Back in the house and hydrating as I type.
Lesson: when a bigger hammer doesn't get the job done, a longer lever is called for.