Doc's right about the type of material you need or recycled crushed concrete is cheap if you can get it. One ton will cover 100 sq.ft. 2 inches deep. Moisten the ground a couple inches deep and tamp it first, then add about 3 inches of material, moisten and tamp. Repeat till level. If you could get a vibrating plate compacter (usually can rent them) that would be better. Work in layers and your base will be as hard as concrete. You could compact all around the perimeter as above (moisten then compact). It sounds like it's worth a little extra effort to prep under the sides. Not sure about taking it apart - I'd try to get it right the first time. Vancouver, BC? Should be highly compactable soils.