In high school, for a science project, I put together a simple timing device, that involved two photovoltaic eyes from Radio Shack, a cheap electronic stopwatch, a breadboard, a bunch of red and black wires, and a couple large lantern batteries for power. Most of the components were just held to the batteries with duct tape. And yes, it looked a lot like a cartoon bomb. I joked how it looked like a cartoon bomb, and so did my parents. My teachers joked about it looking like a cartoon bomb. Somehow, I managed to not get arrested, or have to talk with any police officer that day.
It sure is nice being white some days (ok, just about every day). I suppose it being the early 90s didn't hurt either.
Hell, the school had an engineering teacher. Maybe if you really think you have something slightly suspicious, you give him a call, let him have a look at it. Maybe point out the complete lack of high explosives in it.