That set shows all twenty ways to put two holes into a tetromino. Ali Muniz, by the way, has a page devoted to polyomino covering problems . Thinking more on Fill-Agree by Kadon Enterprises , I asked Michael Reid about all the ways one could drill a hole through a solid L tromino. Start with the tromino, pick any two faces, then connect them with a tunnel. I'm in the process of checking these numbers.
Parity seems to rule out a single closed path, but an open path seems possible.
The set could be put together by drilling 147 cubes and gluing them together. It's a nice simple puzzle. Here is his solution . Henri Picciotto has used polyforms in the classroom for ahile now. He's built a page for them here .