Fastball Flatground Drills

Any one know any good fastball flat-ground drills?

I have my pitchers do a drill i call 4-courners. I draw up boxes the size of the strike zone (I give then a full inch off the plate, bottom of the knees to a few inches above the belt of 6 foot batter) with tape on a wall. I put 5x5 inch boxes in each corner and have them throw 5 pitches to each box from 60 feet at about 85%. after throwing to all 4 boxes i have them throw 1 pitch to each box in any order the choose (calling it beforehand) i chart all of the pitchers on hits to the box and track their progress.