Build a small, lightweight trunk for a bicycle that you can also lock. This cost me about $10.
Step 1: Obtain trunkish object
I used a 16" toolbox from Lowes, which cost me about $8. The brand is "WorkForce" (very tough sounding, no?) and it has a metal clasp, pass-through for a lock, and is made of plastic - all traits it needs for this project.