Neil, you say nearly the size of a bathtub so what is it, metal wood, brick.
Has it drainage holes, if so well and good, pile in a load of drainage crocking, broken brick, gravel. If you have any old compost, soil, soil sand and gravel mix then dump it in remember border plants and bulbs do not need really deep soil, a foot or so of good stuff will do.
Top up with good compost or compost and soil mix, some GC's and supermarkets were selling three bags for £10, mix in some fine grit and some of your own compost if you have any and then plant up.
Watch the watering for a couple of weeks once planted then a mulch of compost now and then will give healthy plants.