$8 cow bell at Harbor Freight.
I've been doing it off and on at work to someone with smaller bells I got on Amazon. Zip-tying them to the underside of his van. I went to his boss ahead of time.. "Look if so and so wants to turn his truck in to the garage because of a noise, don't let him." and explained why.
Once a week I would add another one. He has tons of tools and stuff in the van so it's noisy inside and he runs the A/C all summer so the windows are always up, so while he wasn't hearing it inside, everywhere he went it, every bump he hit, it was sleigh bells. Eventually I added enough that he started to hear them, he mentioned hearing things to someone, so I took them off. The cow bell is now waiting its turn.