To quickly install a garden that will give height to your landscape, why not simply use cinder blocks (hollow, stackable concrete blocks)? They can be placed on any flat surface, even on asphalt, concrete or crushed stone. And all you have to do is to fill the openings with top-quality garden soil (no drainage layer is necessary or even desirable) and install the plants of your choice. Ideally, small to medium-sized plants, given the fairly small growing space offered.
Another possibility is to use cinder blocks to create a raised bed. Simply form a rectangle with the blocks and fill it with soil. You can then garden both inside the garden thus formed as well as planting inside the blocks.
Note that concrete is alkaline but very stable. It is unlikely to leach to any degree and therefore won’t affect the pH of the soil. Concrete dust, though, a result of the manufacturing process, could have an effect, so it would be wise to hose the blocks down before adding soil.