It was fun replicating elements that already existed, but what I really wanted to do was create my own. I found some 1/8" material at home depot (and they can cut it down from the 2'x4' sheet to 12"x24" sheets for me). So my next elements could use that thickness of material.
The first element I designed on my own, I call the "Spike Pit". It's a floor (or wall) that has holes in it.
There's a little lever you can lift to make spikes come out of the holes.