How do you build a aluminum/metal boat that can be no more than 15 cm?

Question

it also needs to be able to hold 50 pennies for @ least 10 seconds.

Answer 1

I'd try a 15cm diameter round boat made out of aluminum foil with enough freeboard to comfortably carry the payload.  

Round gives you the greatest displacement with the least weight of the foil and the greatest stability if loaded correctly.

Answer 2

Try an old soda can. Build the "freeboard" or sides of the boat tall and make sure you have a "fat" or wide hull to displace the weight. Don't cut yourself! Good Luck.