What a great idea. Build a planter out of scrap cinder blocks.
Most people simply take some stiff, small hole galvanized fence screen and bend it into a pocket shape and shove it into the cinder block hole. Friction keeps it in place so don’t trim off all the sharp ends poking out. If you are planting with regular garden soil, it’ll help to push a square of landscape fabric in to line the pocket. Then simply fill with soil and plant seedlings or seeds.
Whether you like the look and go out and buy new cinder blocks or scrounge around and find used ones, this is a great project.