You can pick up a used container for around £400 it may not look very glam but it will be watertight and in good nick which is what you need, there is a vendor in Perry Bar (Birmingham) from whom I purchased one a few years back I think they charges £80 extra for local delivery but its still out there doing its thing, need a bit of grease ever now and then but all in all shipping boxes are a good little storage solution that dont take up much space.