Need to salvage an 3V LED from a happy meal toy or something.
2 plastic soda bottle tops.
A 3V button cell battery, CR2032
A paper clip
Some double-sided foam tape.
And some solder.
Step 1: Cut tops of Soda Bottles
Step 2: Solder the Setup
Then bend the anode sideways and solder on a little .25" piece of paper clip.
Step 3: Assemble and Glue Cap
Mount the penny rig into the cap.
Then place the battery into the cap "-" side down; tilt it around and make adjustments based on how the battery contacts things. When you tilt it toward the LED, the battery should slid and make the "+" connection on the paper clip.
Take the thin cap slice and super glue it onto the base. You may need to file the cap cuts to make the pieces fit together better.
Tape it to the toilet lid and wait for the night.