you can remove the 10 blocks of obsidian and replace it with 10 redstone. You can make obsidian with redstone. here is a vid i verified on 1.8.1 and RC2 :
You might even add a time element to the challenge. have the redstone control a time circuit that will, if your not quick enough, cause the sand to fall. so instead of putting the redstone in a box, you have to recover it from the circuit. then properly build the gate 1 block of obsidian at a time.
Now they have only 1 block of sand that must be duped. just thinking in an evil challenging way.
Wait a sec... YOU EXPECT ME TO HACK REDSTONE INTO THIS?!?! :blink.gif: