An even better idea would be to take your amplifier, fill the bath tub with hot water, power the amp up, then drop it in. After you've finished with that, put the amp on the sidewalk, and beat on it with a hammer.

Uhh, have you ever ridden inside a lifted jeep going 80mph down the highway with the top down in the rain? It isn't that much different than your plan and my amps have survived (well, one of the two does have some issues with the low pass filter). If anything a semi-sealed box under the hood is a better environment as long as the amp has a fan to cool it.

-Mike