I'd put three or so big drain holes.. about a 3/4 inch in diameter in the cab end of the bed. If you make 'em too small, they'll clog easy.. but if you make 'em too big, stuff you're hauling will pour out (corn, dirt, etc)

Altho what I do is park on a hill when it's raining like that.. water pours right out.

an idea is only stupid if you think about it rationally.