Usually plank flooring so it can drain and be replaced easily. You'll likely want to replace with something without gaps.
Very minimal ground clearance for ease of loading stock.
Vents in annoying places if you want windows instead.
I would think a small cargo trailer would be a much better choice!
'Theoretically this level of creeping Orwellian dynamics should ramp up our awareness, but what happens instead is that each alert becomes less and less effective because we're incredibly stupid.' - Jerry Holkins