First what you need to do is make a Square bowl that is 5 blocks tall and at least 20 blocks long and 10 blocks wide. You can make it out of any block that enderman can't move.

Then place blocks such as Grass, Sand, Gravel, Clay, TNT, Pumpkins, Melons, etc. Put them in separate pillars that are 3x3x3.

Once you finish that make sure that you put a roof that prevents rain from hitting the enderman.

Then spawn how ever many enderman you want and put name tags on them to prevent them from despawning.

Also make sure that you set the gamerule to domobspawning False so other mobs don't spawn in the cage.

Now just give them a couple of days to move the blocks. When there done try find out what the sculpture is.

This work great on servers because you can make bets on where the enderman will place blocks or whatever you want it to be.