Aftermarket motion alarm?

Sigh. Kids are playing ball in the culdesac again. I'm tired of asking them to move... they set off a neighbor's car alarm and scattered, which gave me an idea...
So now I'm looking for a loud, motion sensing car alarm I can set in my truck, that will make a huge, obnoxious noise if someone so much as taps my tailgate... any ideas? I've seen a few that are steering wheel locks, but as I've a big cover on my steering wheel, doubt they'll fit. If I took one of those and locked it closed but just left it on the dash, would it still work?
Suggestions on alarms that'd work are welcome. My truck is too old to bother installing an aftermarket alarm system.
So, yes, instead of asking about a noise my truck is making, I'm asking on ways to make it more noise. The irony is not lost on me...
jmc
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