Start by placing stained terracotta around the base of the drop tower, as shown in the first image.

Then add a cross of concrete 5 blocks underneath the base of the drop tower, as shown in second image

Add a block of obsidian underneath each of the blocks of stained terracotta. On the ends of the concrete cross, put a sticky piston facing upwards with a block of slime on the top of it.

Finally, put redstone dust along the concrete cross and add in a form of activation for it. You drop tower is now complete!

Add stone slabs around the bottom, as shown.

Underneath the drop tower put 4 command blocks around the edge of a single block. Change these to be repeating, then add the command from image 2 into them.

Place a button separately to the drop tower (this will act as the activation for it). Coming off of the button, add a 4-tick delay, with a command block on the end. In the command block, enter the command from image 3.

Inside the drop tower add an armour stand, then name it "DropTower" using a name tag.

Add another 4-tick delay coming off of that command block, with another command block after it. In this command block, enter the command from image 1.

Then add a very long delay (the exact length of this depends on the height of your drop tower), after which you need one final command block. In that, enter the command from image 2.

Congratulations! Your drop tower should now be complete!