Large pots, planters, or all manner of re-purposed containers are great way of planting wildflowers in smaller spaces. If it will hold compost and has drainage holes in the bottom (or you can make some) you can probably plant it with wildflowers. Fill the container with potting compost or a 70/30 mixture of compost and soil. Decide whether the area where you want to put the container is in full sun or in partial shade, then plant up with plants that suit.
Some suggestions for a sunny spot are white oxeye daisys, mauve common knapweed, purple selfheal, common toadflax or yellow bird’s foot trefoil for the edges and lilac field scabious. For a spot in partial shade try red campion, selfheal, foxglove, wild strawberry or white hedge bedstraw.
Don’t forget that your containers will require watering regularly in dry weather, especially on hot days.