Draft free,waterproof and if it's not sited on marshland,no need for a floor,although a pallet might be beneficial.If it's going to be a permanent thing,then I'd pour concrete in with a slope towards the door (south-facing). If it's not going to be permanent,then I'd join the posts with rails at ground level and only sink the posts in 6 inches,then you can move it to another spot as and when needed.... Ree