here are my 3d printed 'locks'. more visual than actual real hard locking but anything that looks like its harder than the next car - that MAY help him pass you by and pick the easier car.
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uses mailbox or filing cabling ($5) locks that you find in office stores or hardware stores. there's a cam that is printed and that's what turns and holds under the plastic seat bezel.
mine are in PLA plastic but I plan to have them professional made (shapeways, etc) in ABS so that they will withstand car temp swings. (PLA is just a test print to try this out).
I will probably pick black for the final color and use chrome for the lock, so that its seen but not too inviting.
just an idea. doing nothing is not an option, imho.